diff --git a/.eslintrc.yml b/.eslintrc.yml
index 34c77cabe4..4310cee1bc 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -16,17 +16,22 @@ rules:
     - safe
 overrides:
   - files:
-      - scripts/**/*.js
-      - package-scripts.js
-      - karma.conf.js
-      - .wallaby.js
-      - .eleventy.js
-      - bin/*
-      - lib/cli/**/*.js
-      - test/node-unit/**/*.js
-      - test/integration/options/watch.spec.js
-      - test/integration/helpers.js
-      - lib/growl.js
+      - 'scripts/**/*.js'
+      - 'package-scripts.js'
+      - 'karma.conf.js'
+      - '.wallaby.js'
+      - '.eleventy.js'
+      - 'bin/*'
+      - 'lib/cli/**/*.js'
+      - 'test/node-unit/**/*.js'
+      - 'test/integration/options/watch.spec.js'
+      - 'test/integration/helpers.js'
+      - 'lib/growl.js'
+      - 'lib/buffered-runner.js'
+      - 'lib/worker.js'
+      - 'lib/reporters/buffered.js'
+      - 'lib/serializer.js'
+      - 'test/reporters/buffered.spec.js'
     parserOptions:
       ecmaVersion: 2018
     env:
diff --git a/.mocharc.yml b/.mocharc.yml
index dfb82e07f6..b458eb5423 100644
--- a/.mocharc.yml
+++ b/.mocharc.yml
@@ -5,6 +5,7 @@ global:
   - 'okGlobalC'
   - 'callback*'
 timeout: 1000
+parallel: true
 watch-ignore:
   - '.*'
   - 'docs/_dist/**'
diff --git a/.travis.yml b/.travis.yml
index 6d7a36ebe1..b67b6197fe 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,7 +39,8 @@ jobs:
     - script: COVERAGE=1 npm start test.node
       after_success: npm start coveralls
       name: 'Latest Node.js (with coverage)'
-
+    - script: MOCHA_PARALLEL=0 npm start test.node.unit
+      name: 'Latest Node.js (unit tests in serial mode)'
     - &node
       script: npm start test.node
       node_js: '13'
diff --git a/bin/mocha b/bin/mocha
index bc9aa81a0f..d19d71ffcc 100755
--- a/bin/mocha
+++ b/bin/mocha
@@ -130,8 +130,23 @@ if (Object.keys(nodeArgs).length) {
 
   // terminate children.
   process.on('SIGINT', () => {
-    proc.kill('SIGINT'); // calls runner.abort()
-    proc.kill('SIGTERM'); // if that didn't work, we're probably in an infinite loop, so make it die.
+    // XXX: a previous comment said this would abort the runner, but I can't see that it does
+    // anything with the default runner.
+    debug('main process caught SIGINT');
+    proc.kill('SIGINT');
+    // if running in parallel mode, we will have a proper SIGINT handler, so the below won't
+    // be needed.
+    if (!args.parallel || args.jobs < 2) {
+      // win32 does not support SIGTERM, so use next best thing.
+      if (require('os').platform() === 'win32') {
+        proc.kill('SIGKILL');
+      } else {
+        // using SIGKILL won't cleanly close the output streams, which can result
+        // in cut-off text or a befouled terminal.
+        debug('sending SIGTERM to child process');
+        proc.kill('SIGTERM');
+      }
+    }
   });
 } else {
   debug('running Mocha in-process');
diff --git a/karma.conf.js b/karma.conf.js
index 9337e3d3b8..86975e9d48 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -37,6 +37,10 @@ module.exports = config => {
           .ignore('./lib/esm-utils.js')
           .ignore('path')
           .ignore('supports-color')
+          .ignore('./lib/buffered-runner.js')
+          .ignore('./lib/reporters/buffered.js')
+          .ignore('./lib/serializer.js')
+          .ignore('./lib/worker.js')
           .on('bundled', (err, content) => {
             if (err) {
               throw err;
diff --git a/lib/buffered-runner.js b/lib/buffered-runner.js
new file mode 100644
index 0000000000..56ab2c7272
--- /dev/null
+++ b/lib/buffered-runner.js
@@ -0,0 +1,272 @@
+'use strict';
+
+const allSettled = require('promise.allsettled');
+const os = require('os');
+const Runner = require('./runner');
+const {EVENT_RUN_BEGIN, EVENT_RUN_END} = Runner.constants;
+const debug = require('debug')('mocha:parallel:buffered-runner');
+const workerpool = require('workerpool');
+const {deserialize} = require('./serializer');
+const WORKER_PATH = require.resolve('./worker.js');
+const {setInterval, clearInterval} = global;
+const {createMap, warn} = require('./utils');
+const debugStats = pool => {
+  const {totalWorkers, busyWorkers, idleWorkers, pendingTasks} = pool.stats();
+  debug(
+    '%d/%d busy workers; %d idle; %d tasks queued',
+    busyWorkers,
+    totalWorkers,
+    idleWorkers,
+    pendingTasks
+  );
+};
+
+/**
+ * The interval at which we will display stats for worker processes in debug mode
+ */
+const DEBUG_STATS_INTERVAL = 5000;
+
+const ABORTED = 'ABORTED';
+const IDLE = 'IDLE';
+const ABORTING = 'ABORTING';
+const RUNNING = 'RUNNING';
+const BAILING = 'BAILING';
+const BAILED = 'BAILED';
+const COMPLETE = 'COMPLETE';
+
+const states = createMap({
+  [IDLE]: new Set([RUNNING, ABORTING]),
+  [RUNNING]: new Set([COMPLETE, BAILING, ABORTING]),
+  [COMPLETE]: new Set(),
+  [ABORTED]: new Set(),
+  [ABORTING]: new Set([ABORTED]),
+  [BAILING]: new Set([BAILED, ABORTING]),
+  [BAILED]: new Set([COMPLETE, ABORTING])
+});
+
+/**
+ * This `Runner` delegates tests runs to worker threads.  Does not execute any
+ * {@link Runnable}s by itself!
+ */
+class BufferedRunner extends Runner {
+  constructor(...args) {
+    super(...args);
+
+    let state = IDLE;
+    Object.defineProperty(this, '_state', {
+      get() {
+        return state;
+      },
+      set(newState) {
+        if (states[state].has(newState)) {
+          state = newState;
+        } else {
+          throw new Error(`invalid state transition: ${state} => ${newState}`);
+        }
+      }
+    });
+
+    this.once('EVENT_RUN_END', () => {
+      this._state = COMPLETE;
+    });
+  }
+
+  /**
+   * Runs Mocha tests by creating a thread pool, then delegating work to the
+   * worker threads.
+   *
+   * Each worker receives one file, and as workers become available, they take a
+   * file from the queue and run it. The worker thread execution is treated like
+   * an RPC--it returns a `Promise` containing serialized information about the
+   * run.  The information is processed as it's received, and emitted to a
+   * {@link Reporter}, which is likely listening for these events.
+   *
+   * @todo handle delayed runs?
+   * @param {Function} callback - Called with an exit code corresponding to
+   * number of test failures.
+   * @param {{files: string[], options: Options}} opts - Files to run and
+   * command-line options, respectively.
+   */
+  run(callback, {files, options} = {}) {
+    /**
+     * Listener on `Process.SIGINT` which tries to cleanly terminate the worker pool.
+     */
+    let sigIntListener;
+    // This function should _not_ return a `Promise`; its parent (`Runner#run`)
+    // returns this instance, so this should do the same. However, we want to make
+    // use of `async`/`await`, so we use this IIFE.
+
+    (async () => {
+      /**
+       * This is an interval that outputs stats about the worker pool every so often
+       */
+      let debugInterval;
+
+      /**
+       * @type {WorkerPool}
+       */
+      let pool;
+
+      try {
+        const cpuCount = os.cpus().length;
+        const maxJobs = cpuCount - 1;
+        const jobs = Math.max(1, Math.min(options.jobs || maxJobs, maxJobs));
+        if (maxJobs < 2) {
+          warn(
+            `(Mocha) not enough CPU cores available (${cpuCount}) to run multiple jobs; avoid --parallel on this machine`
+          );
+        } else if (options.jobs && options.jobs > maxJobs) {
+          warn(
+            `(Mocha) ${options.jobs} concurrent jobs requested, but only enough cores available for ${maxJobs}`
+          );
+        }
+        debug(
+          'run(): starting worker pool of size %d, using node args: %s',
+          jobs,
+          process.execArgv.join(' ')
+        );
+        pool = workerpool.pool(WORKER_PATH, {
+          workerType: 'process',
+          maxWorkers: jobs,
+          forkOpts: {execArgv: process.execArgv}
+        });
+
+        sigIntListener = async () => {
+          if (this._state !== ABORTING) {
+            debug('run(): caught a SIGINT');
+            this._state = ABORTING;
+
+            try {
+              debug('run(): shutting down %d (max) workers', jobs);
+              await pool.terminate(true);
+            } catch (err) {
+              console.error(
+                `Error while attempting to force-terminate worker pool: ${err}`
+              );
+            } finally {
+              process.nextTick(() => {
+                debug('run(): imminent death');
+                this._state = ABORTED;
+                process.kill(process.pid, 'SIGINT');
+              });
+            }
+          }
+        };
+
+        process.once('SIGINT', sigIntListener);
+
+        // the "pool proxy" object is essentially just syntactic sugar to call a
+        // worker's procedure as one would a regular function.
+        const poolProxy = await pool.proxy();
+
+        debugInterval = setInterval(
+          () => debugStats(pool),
+          DEBUG_STATS_INTERVAL
+        ).unref();
+
+        // this is set for uncaught exception handling in `Runner#uncaught`
+        this.started = true;
+        this._state = RUNNING;
+
+        this.emit(EVENT_RUN_BEGIN);
+
+        const results = await allSettled(
+          files.map(async file => {
+            debug('run(): enqueueing test file %s', file);
+            try {
+              const result = await poolProxy.run(file, options);
+              if (this._state === BAILED) {
+                // short-circuit after a graceful bail
+                return;
+              }
+              const {failureCount, events} = deserialize(result);
+              debug(
+                'run(): completed run of file %s; %d failures / %d events',
+                file,
+                failureCount,
+                events.length
+              );
+              this.failures += failureCount; // can this ever be non-numeric?
+              /**
+               * If we set this, then we encountered a "bail" flag, and will
+               * terminate the pool once all events have been emitted.
+               */
+              let event = events.shift();
+              while (event) {
+                this.emit(event.eventName, event.data, event.error);
+                if (
+                  this._state !== BAILING &&
+                  event.data &&
+                  event.data._bail &&
+                  (failureCount || event.error)
+                ) {
+                  debug('run(): nonzero failure count & found bail flag');
+                  // we need to let the events complete for this file, as the worker
+                  // should run any cleanup hooks
+                  this._state = BAILING;
+                }
+                event = events.shift();
+              }
+              if (this._state === BAILING) {
+                debug('run(): terminating pool due to "bail" flag');
+                this._state = BAILED;
+                await pool.terminate();
+              }
+            } catch (err) {
+              if (this._state === BAILED || this._state === ABORTING) {
+                debug(
+                  'run(): worker pool terminated with intent; skipping file %s',
+                  file
+                );
+              } else {
+                // this is an uncaught exception
+                debug('run(): encountered uncaught exception: %O', err);
+                if (this.allowUncaught) {
+                  // still have to clean up
+                  this._state = ABORTING;
+                  await pool.terminate(true);
+                }
+                throw err;
+              }
+            } finally {
+              debug('run(): done running file %s', file);
+            }
+          })
+        );
+
+        // note that pool may already be terminated due to --bail
+        await pool.terminate();
+
+        results
+          .filter(({status}) => status === 'rejected')
+          .forEach(({reason}) => {
+            if (this.allowUncaught) {
+              // yep, just the first one.
+              throw reason;
+            }
+            // "rejected" will correspond to uncaught exceptions.
+            // unlike the serial runner, the parallel runner can always recover.
+            this.uncaught(reason);
+          });
+
+        if (this._state === ABORTING) {
+          return;
+        }
+        this.emit(EVENT_RUN_END);
+        debug('run(): completing with failure count %d', this.failures);
+        callback(this.failures);
+      } catch (err) {
+        process.nextTick(() => {
+          debug('run(): throwing uncaught exception');
+          throw err;
+        });
+      } finally {
+        clearInterval(debugInterval);
+        process.removeListener('SIGINT', sigIntListener);
+      }
+    })();
+    return this;
+  }
+}
+
+module.exports = BufferedRunner;
diff --git a/lib/cli/collect-files.js b/lib/cli/collect-files.js
index 61d54ac4b3..37c236d1f5 100644
--- a/lib/cli/collect-files.js
+++ b/lib/cli/collect-files.js
@@ -16,13 +16,7 @@ const utils = require('../utils');
 
 /**
  * Smash together an array of test files in the correct order
- * @param {Object} opts - Options
- * @param {string[]} opts.extension - File extensions to use
- * @param {string[]} opts.spec - Files, dirs, globs to run
- * @param {string[]} opts.ignore - Files, dirs, globs to ignore
- * @param {string[]} opts.file - List of additional files to include
- * @param {boolean} opts.recursive - Find files recursively
- * @param {boolean} opts.sort - Sort test files
+ * @param {FileCollectionOptions} [opts] - Options
  * @returns {string[]} List of files to test
  * @private
  */
@@ -83,3 +77,14 @@ module.exports = ({ignore, extension, file, recursive, sort, spec} = {}) => {
 
   return files;
 };
+
+/**
+ * An object to configure how Mocha gathers test files
+ * @typedef {Object} FileCollectionOptions
+ * @property {string[]} extension - File extensions to use
+ * @property {string[]} spec - Files, dirs, globs to run
+ * @property {string[]} ignore - Files, dirs, globs to ignore
+ * @property {string[]} file - List of additional files to include
+ * @property {boolean} recursive - Find files recursively
+ * @property {boolean} sort - Sort test files
+ */
diff --git a/lib/cli/run-helpers.js b/lib/cli/run-helpers.js
index ebb0bdd071..b4badaedd8 100644
--- a/lib/cli/run-helpers.js
+++ b/lib/cli/run-helpers.js
@@ -10,7 +10,7 @@
 const fs = require('fs');
 const path = require('path');
 const debug = require('debug')('mocha:cli:run:helpers');
-const watchRun = require('./watch-run');
+const {watchRun, watchParallelRun} = require('./watch-run');
 const collectFiles = require('./collect-files');
 const {format} = require('util');
 
@@ -111,24 +111,52 @@ const singleRun = async (mocha, {exit}, fileCollectParams) => {
 };
 
 /**
- * Actually run tests
+ * Collect files and run tests (using `BufferedRunner`).
+ *
+ * This is `async` for consistency.
+ *
  * @param {Mocha} mocha - Mocha instance
- * @param {Object} opts - Command line options
+ * @param {Options} options - Command line options
+ * @param {Object} fileCollectParams - Parameters that control test
+ *   file collection. See `lib/cli/collect-files.js`.
+ * @returns {Promise<BufferedRunner>}
+ * @ignore
+ * @private
+ */
+const parallelRun = async (mocha, options, fileCollectParams) => {
+  const files = collectFiles(fileCollectParams);
+  debug(
+    'executing %d test file(s) across %d concurrent jobs',
+    files.length,
+    options.jobs
+  );
+  mocha.files = files;
+
+  // note that we DO NOT load any files here; this is handled by the worker
+  return mocha.run(options.exit ? exitMocha : exitMochaLater);
+};
+
+/**
+ * Actually run tests.  Delegates to one of four different functions:
+ * - `singleRun`: run tests in serial & exit
+ * - `watchRun`: run tests in serial, rerunning as files change
+ * - `parallelRun`: run tests in parallel & exit
+ * - `watchParallelRun`: run tests in parallel, rerunning as files change
+ * @param {Mocha} mocha - Mocha instance
+ * @param {Options} opts - Command line options
  * @private
- * @returns {Promise}
+ * @returns {Promise<Runner>}
  */
 exports.runMocha = async (mocha, options) => {
   const {
     watch = false,
     extension = [],
-    exit = false,
     ignore = [],
     file = [],
+    parallel = false,
     recursive = false,
     sort = false,
-    spec = [],
-    watchFiles,
-    watchIgnore
+    spec = []
   } = options;
 
   const fileCollectParams = {
@@ -140,11 +168,14 @@ exports.runMocha = async (mocha, options) => {
     spec
   };
 
+  let run;
   if (watch) {
-    watchRun(mocha, {watchFiles, watchIgnore}, fileCollectParams);
+    run = parallel ? watchParallelRun : watchRun;
   } else {
-    await singleRun(mocha, {exit}, fileCollectParams);
+    run = parallel ? parallelRun : singleRun;
   }
+
+  return run(mocha, options, fileCollectParams);
 };
 
 /**
diff --git a/lib/cli/run-option-metadata.js b/lib/cli/run-option-metadata.js
index 4648d9fbfe..da3b7d995d 100644
--- a/lib/cli/run-option-metadata.js
+++ b/lib/cli/run-option-metadata.js
@@ -42,11 +42,12 @@ exports.types = {
     'list-interfaces',
     'list-reporters',
     'no-colors',
+    'parallel',
     'recursive',
     'sort',
     'watch'
   ],
-  number: ['retries'],
+  number: ['retries', 'jobs'],
   string: [
     'config',
     'fgrep',
@@ -75,7 +76,9 @@ exports.aliases = {
   growl: ['G'],
   ignore: ['exclude'],
   invert: ['i'],
+  jobs: ['j'],
   'no-colors': ['C'],
+  parallel: ['p'],
   reporter: ['R'],
   'reporter-option': ['reporter-options', 'O'],
   require: ['r'],
diff --git a/lib/cli/run.js b/lib/cli/run.js
index d024cbb0f2..634ea39e41 100644
--- a/lib/cli/run.js
+++ b/lib/cli/run.js
@@ -24,6 +24,7 @@ const {ONE_AND_DONES, ONE_AND_DONE_ARGS} = require('./one-and-dones');
 const debug = require('debug')('mocha:cli:run');
 const defaults = require('../mocharc');
 const {types, aliases} = require('./run-option-metadata');
+const coreCount = require('os').cpus().length;
 
 /**
  * Logical option groups
@@ -150,6 +151,14 @@ exports.builder = yargs =>
         description: 'Inverts --grep and --fgrep matches',
         group: GROUPS.FILTERS
       },
+      jobs: {
+        description:
+          'Number of concurrent jobs for --parallel; use 1 to run in serial',
+        defaultDescription: '(number of CPU cores - 1)',
+        requiresArg: true,
+        group: GROUPS.RULES,
+        default: Math.max(2, coreCount - 1)
+      },
       'list-interfaces': {
         conflicts: Array.from(ONE_AND_DONE_ARGS),
         description: 'List built-in user interfaces & exit'
@@ -169,6 +178,10 @@ exports.builder = yargs =>
         normalize: true,
         requiresArg: true
       },
+      parallel: {
+        description: 'Run tests in parallel',
+        group: GROUPS.RULES
+      },
       recursive: {
         description: 'Look for tests in subdirectories',
         group: GROUPS.FILES
@@ -271,6 +284,40 @@ exports.builder = yargs =>
         );
       }
 
+      if (argv.parallel) {
+        // yargs.conflicts() can't deal with `--file foo.js --no-parallel`, either
+        if (argv.file) {
+          throw createUnsupportedError(
+            '--parallel runs test files in a non-deterministic order, and is mutually exclusive with --file'
+          );
+        }
+
+        // or this
+        if (argv.sort) {
+          throw createUnsupportedError(
+            '--parallel runs test files in a non-deterministic order, and is mutually exclusive with --sort'
+          );
+        }
+
+        if (argv.reporter === 'progress') {
+          throw createUnsupportedError(
+            '--reporter=progress is mutually exclusive with --parallel'
+          );
+        }
+
+        if (argv.reporter === 'markdown') {
+          throw createUnsupportedError(
+            '--reporter=markdown is mutually exclusive with --parallel'
+          );
+        }
+
+        if (argv.reporter === 'json-stream') {
+          throw createUnsupportedError(
+            '--reporter=json-stream is mutually exclusive with --parallel'
+          );
+        }
+      }
+
       if (argv.compilers) {
         throw createUnsupportedError(
           `--compilers is DEPRECATED and no longer supported.
diff --git a/lib/cli/watch-run.js b/lib/cli/watch-run.js
index b35a906959..d77740dd93 100644
--- a/lib/cli/watch-run.js
+++ b/lib/cli/watch-run.js
@@ -1,5 +1,6 @@
 'use strict';
 
+const debug = require('debug')('mocha:cli:watch');
 const path = require('path');
 const chokidar = require('chokidar');
 const Context = require('../context');
@@ -12,6 +13,42 @@ const collectFiles = require('./collect-files');
  * @private
  */
 
+/**
+ * Run Mocha in parallel "watch" mode
+ * @param {Mocha} mocha - Mocha instance
+ * @param {Object} opts - Options
+ * @param {string[]} [opts.watchFiles] - List of paths and patterns to
+ *   watch. If not provided all files with an extension included in
+ *   `fileColletionParams.extension` are watched. See first argument of
+ *   `chokidar.watch`.
+ * @param {string[]} opts.watchIgnore - List of paths and patterns to
+ *   exclude from watching. See `ignored` option of `chokidar`.
+ * @param {FileCollectionOptions} fileCollectParams - Parameters that control test
+ * @private
+ */
+exports.watchParallelRun = (
+  mocha,
+  {watchFiles, watchIgnore},
+  fileCollectParams
+) => {
+  debug('creating parallel watcher');
+  return createWatcher(mocha, {
+    watchFiles,
+    watchIgnore,
+    beforeRun({mocha}) {
+      mocha.files = collectFiles(fileCollectParams);
+      // in parallel mode, the main Mocha process doesn't actually load the
+      // files. this flag prevents `mocha.run()` from autoloading.
+      mocha.lazyLoadFiles = true;
+      return mocha;
+    },
+    afterRun({watcher}) {
+      blastCache(watcher);
+    },
+    fileCollectParams
+  });
+};
+
 /**
  * Run Mocha in "watch" mode
  * @param {Mocha} mocha - Mocha instance
@@ -22,27 +59,88 @@ const collectFiles = require('./collect-files');
  *   `chokidar.watch`.
  * @param {string[]} opts.watchIgnore - List of paths and patterns to
  *   exclude from watching. See `ignored` option of `chokidar`.
- * @param {Object} fileCollectParams - Parameters that control test
+ * @param {FileCollectionOptions} fileCollectParams - Parameters that control test
  *   file collection. See `lib/cli/collect-files.js`.
- * @param {string[]} fileCollectParams.extension - List of extensions
- *   to watch if `opts.watchFiles` is not given.
  * @private
  */
-module.exports = (mocha, {watchFiles, watchIgnore}, fileCollectParams) => {
+exports.watchRun = (mocha, {watchFiles, watchIgnore}, fileCollectParams) => {
+  debug('creating serial watcher');
+  // list of all test files
+
+  return createWatcher(mocha, {
+    watchFiles,
+    watchIgnore,
+    beforeRun({mocha}) {
+      mocha.unloadFiles();
+
+      // I don't know why we're cloning the root suite.
+      const rootSuite = mocha.suite.clone();
+
+      // this `require` is needed because the require cache has been cleared.  the dynamic
+      // exports set via the below call to `mocha.ui()` won't work properly if a
+      // test depends on this module (see `required-tokens.spec.js`).
+      const Mocha = require('../mocha');
+
+      // ... and now that we've gotten a new module, we need to use it again due
+      // to `mocha.ui()` call
+      const newMocha = new Mocha(mocha.options);
+      // don't know why this is needed
+      newMocha.suite = rootSuite;
+      // nor this
+      newMocha.suite.ctx = new Context();
+
+      // reset the list of files
+      newMocha.files = collectFiles(fileCollectParams);
+
+      // because we've swapped out the root suite (see the `run` inner function
+      // in `createRerunner`), we need to call `mocha.ui()` again to set up the context/globals.
+      newMocha.ui(newMocha.options.ui);
+
+      return newMocha;
+    },
+    afterRun({watcher}) {
+      blastCache(watcher);
+    },
+    fileCollectParams
+  });
+};
+
+/**
+ * Bootstraps a chokidar watcher. Handles keyboard input & signals
+ * @param {Mocha} mocha - Mocha instance
+ * @param {Object} opts
+ * @param {BeforeWatchRun} [opts.beforeRun] - Function to call before
+ * `mocha.run()`
+ * @param {AfterWatchRun} [opts.afterRun] - Function to call after `mocha.run()`
+ * @param {string[]} [opts.watchFiles] - List of paths and patterns to watch. If
+ *   not provided all files with an extension included in
+ *   `fileColletionParams.extension` are watched. See first argument of
+ *   `chokidar.watch`.
+ * @param {string[]} [opts.watchIgnore] - List of paths and patterns to exclude
+ *   from watching. See `ignored` option of `chokidar`.
+ * @param {FileCollectionOptions} opts.fileCollectParams - List of extensions to watch if `opts.watchFiles` is not given.
+ * @returns {FSWatcher}
+ * @ignore
+ * @private
+ */
+const createWatcher = (
+  mocha,
+  {watchFiles, watchIgnore, beforeRun, afterRun, fileCollectParams}
+) => {
   if (!watchFiles) {
     watchFiles = fileCollectParams.extension.map(ext => `**/*.${ext}`);
   }
 
+  debug('ignoring files matching: %s', watchIgnore);
+
   const watcher = chokidar.watch(watchFiles, {
     ignored: watchIgnore,
     ignoreInitial: true
   });
 
-  const rerunner = createRerunner(mocha, () => {
-    getWatchedFiles(watcher).forEach(file => {
-      delete require.cache[file];
-    });
-    mocha.files = collectFiles(fileCollectParams);
+  const rerunner = createRerunner(mocha, watcher, {
+    beforeRun,
+    afterRun
   });
 
   watcher.on('ready', () => {
@@ -53,7 +151,6 @@ module.exports = (mocha, {watchFiles, watchIgnore}, fileCollectParams) => {
     rerunner.scheduleRun();
   });
 
-  console.log();
   hideCursor();
   process.on('exit', () => {
     showCursor();
@@ -74,36 +171,43 @@ module.exports = (mocha, {watchFiles, watchIgnore}, fileCollectParams) => {
       .toLowerCase();
     if (str === 'rs') rerunner.scheduleRun();
   });
+
+  return watcher;
 };
 
 /**
- * Create an object that allows you to rerun tests on the mocha
- * instance. `beforeRun` is called everytime before `mocha.run()` is
- * called.
+ * Create an object that allows you to rerun tests on the mocha instance.
  *
  * @param {Mocha} mocha - Mocha instance
- * @param {function} beforeRun - Called just before `mocha.run()`
+ * @param {FSWatcher} watcher - chokidar `FSWatcher` instance
+ * @param {Object} [opts] - Options!
+ * @param {BeforeWatchRun} [opts.beforeRun] - Function to call before `mocha.run()`
+ * @param {AfterWatchRun} [opts.afterRun] - Function to call after `mocha.run()`
+ * @returns {Rerunner}
+ * @ignore
+ * @private
  */
-const createRerunner = (mocha, beforeRun) => {
+const createRerunner = (mocha, watcher, {beforeRun, afterRun} = {}) => {
   // Set to a `Runner` when mocha is running. Set to `null` when mocha is not
   // running.
   let runner = null;
 
+  // true if a file has changed during a test run
   let rerunScheduled = false;
 
   const run = () => {
-    try {
-      beforeRun();
-      resetMocha(mocha);
-      runner = mocha.run(() => {
-        runner = null;
-        if (rerunScheduled) {
-          rerun();
-        }
-      });
-    } catch (e) {
-      console.log(e.stack);
-    }
+    mocha = beforeRun ? beforeRun({mocha, watcher}) : mocha;
+
+    runner = mocha.run(() => {
+      debug('finished watch run');
+      runner = null;
+      afterRun && afterRun({mocha, watcher});
+      if (rerunScheduled) {
+        rerun();
+      } else {
+        debug('waiting for changes...');
+      }
+    });
   };
 
   const scheduleRun = () => {
@@ -136,32 +240,18 @@ const createRerunner = (mocha, beforeRun) => {
  *
  * @param watcher - Instance of a chokidar watcher
  * @return {string[]} - List of absolute paths
+ * @ignore
+ * @private
  */
 const getWatchedFiles = watcher => {
   const watchedDirs = watcher.getWatched();
-  let watchedFiles = [];
-  Object.keys(watchedDirs).forEach(dir => {
-    watchedFiles = watchedFiles.concat(
-      watchedDirs[dir].map(file => path.join(dir, file))
-    );
-  });
-  return watchedFiles;
-};
-
-/**
- * Reset the internal state of the mocha instance so that tests can be rerun.
- *
- * @param {Mocha} mocha - Mocha instance
- * @private
- */
-const resetMocha = mocha => {
-  mocha.unloadFiles();
-  mocha.suite = mocha.suite.clone();
-  mocha.suite.ctx = new Context();
-  // Registers a callback on `mocha.suite` that wires new context to the DSL
-  // (e.g. `describe`) that is exposed as globals when the test files are
-  // reloaded.
-  mocha.ui(mocha.options.ui);
+  return Object.keys(watchedDirs).reduce(
+    (acc, dir) => [
+      ...acc,
+      ...watchedDirs[dir].map(file => path.join(dir, file))
+    ],
+    []
+  );
 };
 
 /**
@@ -189,3 +279,43 @@ const showCursor = () => {
 const eraseLine = () => {
   process.stdout.write('\u001b[2K');
 };
+
+/**
+ * Blast all of the watched files out of `require.cache`
+ * @param {FSWatcher} watcher - chokidar FSWatcher
+ * @ignore
+ * @private
+ */
+const blastCache = watcher => {
+  const files = getWatchedFiles(watcher);
+  files.forEach(file => {
+    delete require.cache[file];
+  });
+  debug('deleted %d file(s) from the require cache', files.length);
+};
+
+/**
+ * Callback to be run before `mocha.run()` is called.
+ * Optionally, it can return a new `Mocha` instance.
+ * @callback BeforeWatchRun
+ * @private
+ * @param {{mocha: Mocha, watcher: FSWatcher}} options
+ * @returns {Mocha}
+ */
+
+/**
+ * Callback to be run after `mocha.run()` completes.  Typically used to clear
+ * require cache.
+ * @callback AfterWatchRun
+ * @private
+ * @param {{mocha: Mocha, watcher: FSWatcher}} options
+ * @returns {void}
+ */
+
+/**
+ * Object containing run control methods
+ * @typedef {Object} Rerunner
+ * @private
+ * @property {Function} run - Calls `mocha.run()`
+ * @property {Function} scheduleRun - Schedules another call to `run`
+ */
diff --git a/lib/hook.js b/lib/hook.js
index 71440d23d0..4541775c5b 100644
--- a/lib/hook.js
+++ b/lib/hook.js
@@ -44,3 +44,26 @@ Hook.prototype.error = function(err) {
 
   this._error = err;
 };
+
+/**
+ * Returns an object suitable for IPC.
+ * Functions are represented by keys beginning with `$$`.
+ * @returns {Object}
+ */
+Hook.prototype.serialize = function serialize() {
+  return {
+    $$isPending: this.isPending(),
+    $$titlePath: this.titlePath(),
+    ctx: {
+      currentTest: {
+        title: this.ctx && this.ctx.currentTest && this.ctx.currentTest.title
+      }
+    },
+    parent: {
+      root: this.parent.root,
+      title: this.parent.title
+    },
+    title: this.title,
+    type: this.type
+  };
+};
diff --git a/lib/mocha.js b/lib/mocha.js
index 40718d09e9..4c2b173a63 100644
--- a/lib/mocha.js
+++ b/lib/mocha.js
@@ -90,6 +90,8 @@ exports.Test = require('./test');
  * @param {number} [options.slow] - Slow threshold value.
  * @param {number|string} [options.timeout] - Timeout threshold value.
  * @param {string} [options.ui] - Interface name.
+ * @param {boolean} [options.parallel] - Run jobs in parallel
+ * @param {number} [options.jobs] - Max number of worker processes for parallel runs
  */
 function Mocha(options) {
   options = utils.assign({}, mocharc, options || {});
@@ -136,6 +138,14 @@ function Mocha(options) {
       this[opt]();
     }
   }, this);
+
+  if (options.parallel && options.jobs > 1) {
+    this._runner = require('./buffered-runner');
+    this.lazyLoadFiles = true;
+  } else {
+    this._runner = exports.Runner;
+    this.lazyLoadFiles = false;
+  }
 }
 
 /**
@@ -186,20 +196,20 @@ Mocha.prototype.addFile = function(file) {
  * // Use XUnit reporter and direct its output to file
  * mocha.reporter('xunit', { output: '/path/to/testspec.xunit.xml' });
  */
-Mocha.prototype.reporter = function(reporter, reporterOptions) {
-  if (typeof reporter === 'function') {
-    this._reporter = reporter;
+Mocha.prototype.reporter = function(reporterName, reporterOptions) {
+  if (typeof reporterName === 'function') {
+    this._reporter = reporterName;
   } else {
-    reporter = reporter || 'spec';
-    var _reporter;
+    reporterName = reporterName || 'spec';
+    var reporter;
     // Try to load a built-in reporter.
-    if (builtinReporters[reporter]) {
-      _reporter = builtinReporters[reporter];
+    if (builtinReporters[reporterName]) {
+      reporter = builtinReporters[reporterName];
     }
     // Try to load reporters from process.cwd() and node_modules
-    if (!_reporter) {
+    if (!reporter) {
       try {
-        _reporter = require(reporter);
+        reporter = require(reporterName);
       } catch (err) {
         if (
           err.code !== 'MODULE_NOT_FOUND' ||
@@ -207,31 +217,31 @@ Mocha.prototype.reporter = function(reporter, reporterOptions) {
         ) {
           // Try to load reporters from a path (absolute or relative)
           try {
-            _reporter = require(path.resolve(process.cwd(), reporter));
+            reporter = require(path.resolve(process.cwd(), reporterName));
           } catch (_err) {
             _err.code !== 'MODULE_NOT_FOUND' ||
             _err.message.indexOf('Cannot find module') !== -1
-              ? console.warn(sQuote(reporter) + ' reporter not found')
+              ? console.warn(sQuote(reporterName) + ' reporter not found')
               : console.warn(
-                  sQuote(reporter) +
+                  sQuote(reporterName) +
                     ' reporter blew up with error:\n' +
                     err.stack
                 );
           }
         } else {
           console.warn(
-            sQuote(reporter) + ' reporter blew up with error:\n' + err.stack
+            sQuote(reporterName) + ' reporter blew up with error:\n' + err.stack
           );
         }
       }
     }
-    if (!_reporter) {
+    if (!reporter) {
       throw createInvalidReporterError(
-        'invalid reporter ' + sQuote(reporter),
-        reporter
+        'invalid reporter ' + sQuote(reporterName),
+        reporterName
       );
     }
-    this._reporter = _reporter;
+    this._reporter = reporter;
   }
   this.options.reporterOption = reporterOptions;
   // alias option name is used in public reporters xunit/tap/progress
@@ -340,7 +350,7 @@ Mocha.prototype.loadFiles = function(fn) {
 Mocha.prototype.loadFilesAsync = function() {
   var self = this;
   var suite = this.suite;
-  this.loadAsync = true;
+  this.lazyLoadFiles = true;
 
   if (!esmUtils) {
     return new Promise(function(resolve) {
@@ -827,13 +837,13 @@ Object.defineProperty(Mocha.prototype, 'version', {
  * mocha.run(failures => process.exitCode = failures ? 1 : 0);
  */
 Mocha.prototype.run = function(fn) {
-  if (this.files.length && !this.loadAsync) {
+  if (this.files.length && !this.lazyLoadFiles) {
     this.loadFiles();
   }
   var suite = this.suite;
   var options = this.options;
   options.files = this.files;
-  var runner = new exports.Runner(suite, options.delay);
+  var runner = new this._runner(suite, options.delay);
   createStatsCollector(runner);
   var reporter = new this._reporter(runner, options);
   runner.checkLeaks = options.checkLeaks === true;
@@ -866,5 +876,5 @@ Mocha.prototype.run = function(fn) {
     }
   }
 
-  return runner.run(done);
+  return runner.run(done, {files: this.files, options: options});
 };
diff --git a/lib/reporters/buffered.js b/lib/reporters/buffered.js
new file mode 100644
index 0000000000..1bbca61d36
--- /dev/null
+++ b/lib/reporters/buffered.js
@@ -0,0 +1,128 @@
+'use strict';
+/**
+ * @module Buffered
+ */
+/**
+ * Module dependencies.
+ */
+
+const {
+  EVENT_SUITE_BEGIN,
+  EVENT_SUITE_END,
+  EVENT_TEST_FAIL,
+  EVENT_TEST_PASS,
+  EVENT_TEST_PENDING,
+  EVENT_TEST_BEGIN,
+  EVENT_TEST_END,
+  EVENT_TEST_RETRY,
+  EVENT_DELAY_BEGIN,
+  EVENT_DELAY_END,
+  EVENT_HOOK_BEGIN,
+  EVENT_HOOK_END,
+  EVENT_RUN_END
+} = require('../runner').constants;
+const {SerializableEvent, SerializableWorkerResult} = require('../serializer');
+const debug = require('debug')('mocha:reporters:buffered');
+const Base = require('./base');
+
+/**
+ * List of events to listen to; these will be buffered and sent
+ * when `Mocha#run` is complete (via {@link Buffered#done}).
+ */
+const EVENT_NAMES = [
+  EVENT_SUITE_BEGIN,
+  EVENT_SUITE_END,
+  EVENT_TEST_BEGIN,
+  EVENT_TEST_PENDING,
+  EVENT_TEST_FAIL,
+  EVENT_TEST_PASS,
+  EVENT_TEST_RETRY,
+  EVENT_TEST_END,
+  EVENT_HOOK_BEGIN,
+  EVENT_HOOK_END
+];
+
+/**
+ * Like {@link EVENT_NAMES}, except we expect these events to only be emitted
+ * by the `Runner` once.
+ */
+const ONCE_EVENT_NAMES = [EVENT_DELAY_BEGIN, EVENT_DELAY_END];
+
+/**
+ * The `Buffered` reporter is for use by concurrent runs. Instead of outputting
+ * to `STDOUT`, etc., it retains a list of events it receives and hands these
+ * off to the callback passed into {@link Mocha#run}. That callback will then
+ * return the data to the main process.
+ */
+class Buffered extends Base {
+  /**
+   * Listens for {@link Runner} events and retains them in an `events` instance prop.
+   * @param {Runner} runner
+   */
+  constructor(runner, opts) {
+    super(runner, opts);
+
+    /**
+     * Retained list of events emitted from the {@link Runner} instance.
+     * @type {BufferedEvent[]}
+     * @memberOf Buffered
+     */
+    const events = (this.events = []);
+
+    /**
+     * mapping of event names to listener functions we've created,
+     * so we can cleanly _remove_ them from the runner once it's completed.
+     */
+    const listeners = new Map();
+
+    /**
+     * Creates a listener for event `eventName` and adds it to the `listeners`
+     * map. This is a defensive measure, so that we don't a) leak memory or b)
+     * remove _other_ listeners that may not be associated with this reporter.
+     * @param {string} eventName - Event name
+     */
+    const createListener = eventName =>
+      listeners
+        .set(eventName, (runnable, err) => {
+          events.push(SerializableEvent.create(eventName, runnable, err));
+        })
+        .get(eventName);
+
+    EVENT_NAMES.forEach(evt => {
+      runner.on(evt, createListener(evt));
+    });
+    ONCE_EVENT_NAMES.forEach(evt => {
+      runner.once(evt, createListener(evt));
+    });
+
+    runner.once(EVENT_RUN_END, () => {
+      debug('received EVENT_RUN_END');
+      listeners.forEach((listener, evt) => {
+        runner.removeListener(evt, listener);
+        listeners.delete(evt);
+      });
+    });
+  }
+
+  /**
+   * Calls the {@link Mocha#run} callback (`callback`) with the test failure
+   * count and the array of {@link BufferedEvent} objects. Resets the array.
+   * @param {number} failures - Number of failed tests
+   * @param {Function} callback - The callback passed to {@link Mocha#run}.
+   */
+  done(failures, callback) {
+    callback(SerializableWorkerResult.create(this.events, failures));
+    this.events = []; // defensive
+  }
+}
+
+/**
+ * Serializable event data from a `Runner`.  Keys of the `data` property
+ * beginning with `__` will be converted into a function which returns the value
+ * upon deserialization.
+ * @typedef {Object} BufferedEvent
+ * @property {string} name - Event name
+ * @property {object} data - Event parameters
+ */
+
+module.exports = Buffered;
diff --git a/lib/reporters/landing.js b/lib/reporters/landing.js
index 0be3837d7b..03c9acb1af 100644
--- a/lib/reporters/landing.js
+++ b/lib/reporters/landing.js
@@ -56,11 +56,12 @@ function Landing(runner, options) {
 
   var self = this;
   var width = (Base.window.width * 0.75) | 0;
-  var total = runner.total;
   var stream = process.stdout;
+
   var plane = color('plane', '✈');
   var crashed = -1;
   var n = 0;
+  var total = 0;
 
   function runway() {
     var buf = Array(width).join('-');
@@ -74,8 +75,7 @@ function Landing(runner, options) {
 
   runner.on(EVENT_TEST_END, function(test) {
     // check if the plane crashed
-    var col = crashed === -1 ? ((width * ++n) / total) | 0 : crashed;
-
+    var col = crashed === -1 ? ((width * ++n) / ++total) | 0 : crashed;
     // show the crash
     if (test.state === STATE_FAILED) {
       plane = color('plane crash', '✈');
diff --git a/lib/reporters/tap.js b/lib/reporters/tap.js
index 12257a745f..aa79fb19f1 100644
--- a/lib/reporters/tap.js
+++ b/lib/reporters/tap.js
@@ -50,9 +50,7 @@ function TAP(runner, options) {
   this._producer = createProducer(tapVersion);
 
   runner.once(EVENT_RUN_BEGIN, function() {
-    var ntests = runner.grepTotal(runner.suite);
     self._producer.writeVersion();
-    self._producer.writePlan(ntests);
   });
 
   runner.on(EVENT_TEST_END, function() {
@@ -204,6 +202,7 @@ TAPProducer.prototype.writeEpilogue = function(stats) {
   println('# pass ' + stats.passes);
   // :TBD: Why are we not showing pending results?
   println('# fail ' + stats.failures);
+  this.writePlan(stats.passes + stats.failures + stats.pending);
 };
 
 /**
diff --git a/lib/runner.js b/lib/runner.js
index aabffda96a..7df7806c34 100644
--- a/lib/runner.js
+++ b/lib/runner.js
@@ -880,10 +880,11 @@ Runner.prototype.uncaughtEnd = function uncaughtEnd(err) {
  *
  * @public
  * @memberof Runner
- * @param {Function} fn
+ * @param {Function} fn - Callback when finished
+ * @param {{files: string[], options: Options}} [opts] - For subclasses
  * @return {Runner} Runner instance.
  */
-Runner.prototype.run = function(fn) {
+Runner.prototype.run = function(fn, opts) {
   var self = this;
   var rootSuite = this.suite;
 
diff --git a/lib/serializer.js b/lib/serializer.js
new file mode 100644
index 0000000000..db2166a415
--- /dev/null
+++ b/lib/serializer.js
@@ -0,0 +1,392 @@
+'use strict';
+
+const {type} = require('./utils');
+const {createInvalidArgumentTypeError} = require('./errors');
+const debug = require('debug')('mocha:serializer');
+
+const SERIALIZABLE_RESULT_NAME = 'SerializableWorkerResult';
+const SERIALIZABLE_TYPES = new Set(['object', 'array', 'function', 'error']);
+
+/**
+ * The serializable result of a test file run from a worker.
+ */
+class SerializableWorkerResult {
+  /**
+   * Creates instance props; of note, the `__type` prop.
+   *
+   * Note that the failure count is _redundant_ and could be derived from the
+   * list of events; but since we're already doing the work, might as well use
+   * it.
+   * @param {SerializableEvent[]} [events=[]] - Events to eventually serialize
+   * @param {number} [failureCount=0] - Failure count
+   */
+  constructor(events = [], failureCount = 0) {
+    /**
+     * The number of failures in this run
+     * @type {number}
+     */
+    this.failureCount = failureCount;
+    /**
+     * All relevant events emitted from the {@link Runner}.
+     * @type {SerializableEvent[]}
+     */
+    this.events = events;
+
+    /**
+     * Symbol-like value needed to distinguish when attempting to deserialize
+     * this object (once it's been received over IPC).
+     * @type {Readonly<"SerializableWorkerResult">}
+     */
+    Object.defineProperty(this, '__type', {
+      value: SERIALIZABLE_RESULT_NAME,
+      enumerable: true,
+      writable: false
+    });
+  }
+
+  /**
+   * Instantiates a new {@link SerializableWorkerResult}.
+   * @param {...any} args - Args to constructor
+   * @returns {SerilizableWorkerResult}
+   */
+  static create(...args) {
+    return new SerializableWorkerResult(...args);
+  }
+
+  /**
+   * Serializes each {@link SerializableEvent} in our `events` prop;
+   * makes this object read-only.
+   * @returns {Readonly<SerializableWorkerResult>}
+   */
+  serialize() {
+    this.events.forEach(event => {
+      event.serialize();
+    });
+    return Object.freeze(this);
+  }
+
+  /**
+   * Deserializes a {@link SerializedWorkerResult} into something reporters can
+   * use; calls {@link SerializableEvent.deserialize} on each item in its
+   * `events` prop.
+   * @param {SerializedWorkerResult} obj
+   * @returns {SerializedWorkerResult}
+   */
+  static deserialize(obj) {
+    obj.events.forEach(event => {
+      SerializableEvent.deserialize(event);
+    });
+    return obj;
+  }
+
+  /**
+   * Returns `true` if this is a {@link SerializedWorkerResult} or a
+   * {@link SerializableWorkerResult}.
+   * @param {*} value - A value to check
+   * @returns {boolean} If true, it's deserializable
+   */
+  static isSerializedWorkerResult(value) {
+    return (
+      value instanceof SerializableWorkerResult ||
+      (type(value) === 'object' && value.__type === SERIALIZABLE_RESULT_NAME)
+    );
+  }
+}
+
+/**
+ * Represents an event, emitted by a {@link Runner}, which is to be transmitted
+ * over IPC.
+ *
+ * Due to the contents of the event data, it's not possible to send them
+ * verbatim. When received by the main process--and handled by reporters--these
+ * objects are expected to contain {@link Runnable} instances.  This class
+ * provides facilities to perform the translation via serialization and
+ * deserialization.
+ */
+class SerializableEvent {
+  /**
+   * Constructs a `SerializableEvent`, throwing if we receive unexpected data.
+   *
+   * Practically, events emitted from `Runner` have a minumum of zero (0)
+   * arguments-- (for example, {@link Runnable.constants.EVENT_RUN_BEGIN}) and a
+   * maximum of two (2) (for example,
+   * {@link Runnable.constants.EVENT_TEST_FAIL}, where the second argument is an
+   * `Error`).  The first argument, if present, is a {@link Runnable}. This
+   * constructor's arguments adhere to this convention.
+   * @param {string} eventName - A non-empty event name.
+   * @param {any} [originalValue] - Some data. Corresponds to extra arguments
+   * passed to `EventEmitter#emit`.
+   * @param {Error} [originalError] - An error, if there's an error.
+   * @throws If `eventName` is empty, or `originalValue` is a non-object.
+   */
+  constructor(eventName, originalValue, originalError) {
+    if (!eventName) {
+      throw new Error('expected a non-empty `eventName` string argument');
+    }
+    /**
+     * The event name.
+     * @memberof SerializableEvent
+     */
+    this.eventName = eventName;
+    const originalValueType = type(originalValue);
+    if (originalValueType !== 'object' && originalValueType !== 'undefined') {
+      throw new Error(
+        `expected object, received [${originalValueType}]: ${originalValue}`
+      );
+    }
+    /**
+     * An error, if present.
+     * @memberof SerializableEvent
+     */
+    Object.defineProperty(this, 'originalError', {
+      value: originalError,
+      enumerable: false
+    });
+
+    /**
+     * The raw value.
+     *
+     * We don't want this value sent via IPC; making it non-enumerable will do that.
+     *
+     * @memberof SerializableEvent
+     */
+    Object.defineProperty(this, 'originalValue', {
+      value: originalValue,
+      enumerable: false
+    });
+  }
+
+  /**
+   * In case you hated using `new` (I do).
+   *
+   * @param  {...any} args - Args for {@link SerializableEvent#constructor}.
+   * @returns {SerializableEvent} A new `SerializableEvent`
+   */
+  static create(...args) {
+    return new SerializableEvent(...args);
+  }
+
+  /**
+   * Used internally by {@link SerilizableEvent#serialize}.
+   * @ignore
+   * @param {Array<object|string>} pairs - List of parent/key tuples to process; modified in-place. This JSDoc type is an approximation
+   * @param {object} parent - Some parent object
+   * @param {string} key - Key to inspect
+   * @param {WeakSet<Object>} seenObjects - For avoiding circular references
+   */
+  static _serialize(pairs, parent, key, seenObjects) {
+    let value = parent[key];
+    if (seenObjects.has(value)) {
+      parent[key] = Object.create(null);
+      return;
+    }
+    if (type(value) === 'error' || value instanceof Error) {
+      // we need to reference the stack prop b/c it's lazily-loaded.
+      // `__type` is necessary for deserialization to create an `Error` later.
+      // `message` is apparently not enumerable, so we must handle it specifically.
+      value = Object.assign(Object.create(null), value, {
+        stack: value.stack,
+        message: value.message,
+        __type: 'Error'
+      });
+      parent[key] = value;
+      // after this, the result of type(value) will be `object`, and we'll throw
+      // whatever other junk is in the original error into the new `value`.
+    }
+    switch (type(value)) {
+      case 'object':
+        if (type(value.serialize) === 'function') {
+          parent[key] = value.serialize();
+        } else {
+          // by adding props to the `pairs` array, we will process it further
+          pairs.push(
+            ...Object.keys(value)
+              .filter(key => SERIALIZABLE_TYPES.has(type(value[key])))
+              .map(key => [value, key])
+          );
+        }
+        break;
+      case 'function':
+        // we _may_ want to dig in to functions for some assertion libraries
+        // that might put a usable property on a function.
+        // for now, just zap it.
+        delete parent[key];
+        break;
+      case 'array':
+        pairs.push(
+          ...value
+            .filter(value => SERIALIZABLE_TYPES.has(type(value)))
+            .map((value, index) => [value, index])
+        );
+        break;
+    }
+  }
+
+  /**
+   * Modifies this object *in place* (for theoretical memory consumption &
+   * performance reasons); serializes `SerializableEvent#originalValue` (placing
+   * the result in `SerializableEvent#data`) and `SerializableEvent#error`.
+   * Freezes this object. The result is an object that can be transmitted over
+   * IPC.
+   * If this quickly becomes unmaintainable, we will want to move towards immutable
+   * objects post-haste.
+   */
+  serialize() {
+    // given a parent object and a key, inspect the value and decide whether
+    // to replace it, remove it, or add it to our `pairs` array to further process.
+    // this is recursion in loop form.
+    const originalValue = this.originalValue;
+    const result = Object.assign(Object.create(null), {
+      data:
+        type(originalValue) === 'object' &&
+        type(originalValue.serialize) === 'function'
+          ? originalValue.serialize()
+          : originalValue,
+      error: this.originalError
+    });
+
+    const pairs = Object.keys(result).map(key => [result, key]);
+    const seenObjects = new WeakSet();
+
+    let pair;
+    while ((pair = pairs.shift())) {
+      SerializableEvent._serialize(pairs, ...pair, seenObjects);
+      seenObjects.add(pair[0]);
+    }
+
+    this.data = result.data;
+    this.error = result.error;
+
+    return Object.freeze(this);
+  }
+
+  /**
+   * Used internally by {@link SerializableEvent.deserialize}; creates an `Error`
+   * from an `Error`-like (serialized) object
+   * @ignore
+   * @param {Object} value - An Error-like value
+   * @returns {Error} Real error
+   */
+  static _deserializeError(value) {
+    const error = new Error(value.message);
+    error.stack = value.stack;
+    Object.assign(error, value);
+    delete error.__type;
+    return error;
+  }
+
+  /**
+   * Used internally by {@link SerializableEvent.deserialize}; recursively
+   * deserializes an object in-place.
+   * @param {object|Array} parent - Some object or array
+   * @param {string|number} key - Some prop name or array index within `parent`
+   */
+  static _deserializeObject(parent, key) {
+    if (key === '__proto__') {
+      delete parent[key];
+      return;
+    }
+    const value = parent[key];
+    // keys beginning with `$$` are converted into functions returning the value
+    // and renamed, stripping the `$$` prefix.
+    // functions defined this way cannot be array members!
+    if (type(key) === 'string' && key.startsWith('$$')) {
+      const newKey = key.slice(2);
+      parent[newKey] = () => value;
+      delete parent[key];
+      key = newKey;
+    }
+    if (type(value) === 'array') {
+      value.forEach((_, idx) => {
+        SerializableEvent._deserializeObject(value, idx);
+      });
+    } else if (type(value) === 'object') {
+      if (value.__type === 'Error') {
+        parent[key] = SerializableEvent._deserializeError(value);
+      } else {
+        Object.keys(value).forEach(key => {
+          SerializableEvent._deserializeObject(value, key);
+        });
+      }
+    }
+  }
+
+  /**
+   * Deserialize value returned from a worker into something more useful.
+   * Does not return the same object.
+   * @todo - do this in a loop instead of with recursion (if necessary)
+   * @param {SerializedEvent} obj - Object returned from worker
+   * @returns {SerializedEvent} Deserialized result
+   */
+  static deserialize(obj) {
+    if (!obj) {
+      throw createInvalidArgumentTypeError('Expected value', obj);
+    }
+
+    obj = Object.assign(Object.create(null), obj);
+
+    if (obj.data) {
+      Object.keys(obj.data).forEach(key => {
+        SerializableEvent._deserializeObject(obj.data, key);
+      });
+    }
+
+    if (obj.error) {
+      obj.error = SerializableEvent._deserializeError(obj.error);
+    }
+
+    return obj;
+  }
+}
+
+/**
+ * "Serializes" a value for transmission over IPC as a message.
+ *
+ * If value is an object and has a `serialize()` method, call that method; otherwise return the object and hope for the best.
+ *
+ * @param {*} [value] - A value to serialize
+ */
+exports.serialize = function serialize(value) {
+  const result =
+    type(value) === 'object' && type(value.serialize) === 'function'
+      ? value.serialize()
+      : value;
+  debug('serialized: %O', result);
+  return result;
+};
+
+/**
+ * "Deserializes" a "message" received over IPC.
+ *
+ * This could be expanded with other objects that need deserialization,
+ * but at present time we only care about {@link SerializableWorkerResult} objects.
+ *
+ * @param {*} [value] - A "message" to deserialize
+ */
+exports.deserialize = function deserialize(value) {
+  const result = SerializableWorkerResult.isSerializedWorkerResult(value)
+    ? SerializableWorkerResult.deserialize(value)
+    : value;
+  debug('deserialized: %O', result);
+  return result;
+};
+
+exports.SerializableEvent = SerializableEvent;
+exports.SerializableWorkerResult = SerializableWorkerResult;
+
+/**
+ * The result of calling `SerializableEvent.serialize`, as received
+ * by the deserializer.
+ * @typedef {Object} SerializedEvent
+ * @property {object?} data - Optional serialized data
+ * @property {object?} error - Optional serialized `Error`
+ */
+
+/**
+ * The result of calling `SerializableWorkerResult.serialize` as received
+ * by the deserializer.
+ * @typedef {Object} SerializedWorkerResult
+ * @property {number} failureCount - Number of failures
+ * @property {SerializedEvent[]} events - Serialized events
+ * @property {"SerializedWorkerResult"} __type - Symbol-like to denote the type of object this is
+ */
diff --git a/lib/suite.js b/lib/suite.js
index 191d946b50..a8301cc809 100644
--- a/lib/suite.js
+++ b/lib/suite.js
@@ -549,6 +549,21 @@ Suite.prototype.cleanReferences = function cleanReferences() {
   }
 };
 
+/**
+ * Returns an object suitable for IPC.
+ * Functions are represented by keys beginning with `$$`.
+ * @returns {Object}
+ */
+Suite.prototype.serialize = function serialize() {
+  return {
+    _bail: this._bail,
+    $$fullTitle: this.fullTitle(),
+    $$isPending: this.isPending(),
+    root: this.root,
+    title: this.title
+  };
+};
+
 var constants = utils.defineConstants(
   /**
    * {@link Suite}-related constants.
diff --git a/lib/test.js b/lib/test.js
index 87f1ccce7d..e5c4583157 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -71,3 +71,29 @@ Test.prototype.clone = function() {
   test.ctx = this.ctx;
   return test;
 };
+
+/**
+ * Returns an object suitable for IPC.
+ * Functions are represented by keys beginning with `$$`.
+ * @returns {Object}
+ */
+Test.prototype.serialize = function serialize() {
+  return {
+    $$currentRetry: this._currentRetry,
+    $$fullTitle: this.fullTitle(),
+    $$isPending: this.pending,
+    $$retriedTest: this._retriedTest || null,
+    $$slow: this._slow,
+    $$titlePath: this.titlePath(),
+    body: this.body,
+    duration: this.duration,
+    err: this.err,
+    parent: {
+      $$fullTitle: this.parent.fullTitle()
+    },
+    speed: this.speed,
+    state: this.state,
+    title: this.title,
+    type: this.type
+  };
+};
diff --git a/lib/worker.js b/lib/worker.js
new file mode 100644
index 0000000000..32cfaa246d
--- /dev/null
+++ b/lib/worker.js
@@ -0,0 +1,114 @@
+'use strict';
+
+const {createInvalidArgumentTypeError} = require('./errors');
+const workerpool = require('workerpool');
+const Mocha = require('./mocha');
+const {
+  handleRequires,
+  validatePlugin,
+  loadRootHooks
+} = require('./cli/run-helpers');
+const debug = require('debug')(`mocha:parallel:worker:${process.pid}`);
+const {serialize} = require('./serializer');
+const {setInterval, clearInterval} = global;
+
+const BUFFERED_REPORTER_PATH = require.resolve('./reporters/buffered');
+
+let rootHooks;
+
+if (workerpool.isMainThread) {
+  throw new Error(
+    'This script is intended to be run as a worker (by the `workerpool` package).'
+  );
+}
+
+/**
+ * Initializes some stuff on the first call to {@link run}.
+ *
+ * Handles `--require` and `--ui`.  Does _not_ handle `--reporter`,
+ * as only the `Buffered` reporter is used.
+ *
+ * **This function only runs once per worker**; it overwrites itself with a no-op
+ * before returning.
+ *
+ * @param {Options} argv - Command-line options
+ */
+let bootstrap = async argv => {
+  const rawRootHooks = handleRequires(argv.require);
+  rootHooks = await loadRootHooks(rawRootHooks);
+  validatePlugin(argv, 'ui', Mocha.interfaces);
+  bootstrap = () => {};
+};
+
+/**
+ * Runs a single test file in a worker thread.
+ * @param {string} filepath - Filepath of test file
+ * @param {Options} [argv] - Parsed command-line options object
+ * @returns {Promise<{failures: number, events: BufferedEvent[]}>} - Test
+ * failure count and list of events.
+ */
+async function run(filepath, argv = {ui: 'bdd'}) {
+  if (!filepath) {
+    throw createInvalidArgumentTypeError(
+      'Expected a non-empty "filepath" argument',
+      'file',
+      'string'
+    );
+  }
+
+  debug('run(): running test file %s', filepath);
+
+  const opts = Object.assign(argv, {
+    // workers only use the `Buffered` reporter.
+    reporter: BUFFERED_REPORTER_PATH,
+    // if this was true, it would cause infinite recursion.
+    parallel: false
+  });
+
+  await bootstrap(opts);
+
+  opts.rootHooks = rootHooks;
+
+  const mocha = new Mocha(opts).addFile(filepath);
+
+  try {
+    await mocha.loadFilesAsync();
+  } catch (err) {
+    debug('run(): could not load file %s: %s', filepath, err);
+    throw err;
+  }
+
+  return new Promise((resolve, reject) => {
+    const t = setInterval(() => {
+      debug('run(): still running %s...', filepath);
+    }, 5000).unref();
+    mocha.run(result => {
+      // Runner adds these; if we don't remove them, we'll get a leak.
+      process.removeAllListeners('uncaughtException');
+
+      try {
+        const serialized = serialize(result);
+        debug(
+          'run(): completed run with %d test failures; returning to main process',
+          typeof result.failures === 'number' ? result.failures : 0
+        );
+        resolve(serialized);
+      } catch (err) {
+        // TODO: figure out exactly what the sad path looks like here.
+        // rejection should only happen if an error is "unrecoverable"
+        debug('run(): serialization failed; rejecting: %O', err);
+        reject(err);
+      } finally {
+        clearInterval(t);
+      }
+    });
+  });
+}
+
+// this registers the `run` function.
+workerpool.worker({run});
+
+debug('started worker process');
+
+// for testing
+exports.run = run;
diff --git a/package-lock.json b/package-lock.json
index a7c70cbcdf..3d18e37a3e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -978,7 +978,7 @@
     },
     "array-equal": {
       "version": "1.0.0",
-      "resolved": "http://registry.npmjs.org/array-equal/-/array-equal-1.0.0.tgz",
+      "resolved": "https://registry.npmjs.org/array-equal/-/array-equal-1.0.0.tgz",
       "integrity": "sha1-jCpe8kcv2ep0KwTHenUJO6J1fJM=",
       "dev": true
     },
@@ -1039,6 +1039,17 @@
         "es-abstract": "^1.17.0-next.1"
       }
     },
+    "array.prototype.map": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/array.prototype.map/-/array.prototype.map-1.0.2.tgz",
+      "integrity": "sha512-Az3OYxgsa1g7xDYp86l0nnN4bcmuEITGe1rbdEBVkrqkzMgDcbdQ2R7r41pNzti+4NMces3H8gMmuioZUilLgw==",
+      "requires": {
+        "define-properties": "^1.1.3",
+        "es-abstract": "^1.17.0-next.1",
+        "es-array-method-boxes-properly": "^1.0.0",
+        "is-string": "^1.0.4"
+      }
+    },
     "arraybuffer.slice": {
       "version": "0.0.7",
       "resolved": "https://registry.npmjs.org/arraybuffer.slice/-/arraybuffer.slice-0.0.7.tgz",
@@ -1838,7 +1849,7 @@
     },
     "bl": {
       "version": "1.2.2",
-      "resolved": "http://registry.npmjs.org/bl/-/bl-1.2.2.tgz",
+      "resolved": "https://registry.npmjs.org/bl/-/bl-1.2.2.tgz",
       "integrity": "sha512-e8tQYnZodmebYDWGH7KMRvtzKXaJHx3BbilrgZCfvyLUYdKpK1t5PSPmpkny/SgiTSCnjfLW7v5rlONXVFkQEA==",
       "dev": true,
       "requires": {
@@ -1861,7 +1872,7 @@
     "bn.js": {
       "version": "4.11.8",
       "resolved": "https://registry.npmjs.org/bn.js/-/bn.js-4.11.8.tgz",
-      "integrity": "sha512-ItfYfPLkWHUjckQCk8xC+LwxgK8NYcXywGigJgSwOP8Y2iyWT4f2vsZnoOXTTbo+o5yXmIUJ4gn5538SO5S3gA==",
+      "integrity": "sha1-LN4J617jQfSEdGuwMJsyU7GxRC8=",
       "dev": true
     },
     "body-parser": {
@@ -3106,7 +3117,7 @@
     },
     "browserify-aes": {
       "version": "1.2.0",
-      "resolved": "http://registry.npmjs.org/browserify-aes/-/browserify-aes-1.2.0.tgz",
+      "resolved": "https://registry.npmjs.org/browserify-aes/-/browserify-aes-1.2.0.tgz",
       "integrity": "sha512-+7CHXqGuspUn/Sl5aO7Ea0xWGAtETPXNSAjHo48JfLdPWcMng33Xe4znFvQweqc/uzk5zSOI3H52CYnjCfb5hA==",
       "dev": true,
       "requires": {
@@ -3149,7 +3160,7 @@
     },
     "browserify-rsa": {
       "version": "4.0.1",
-      "resolved": "http://registry.npmjs.org/browserify-rsa/-/browserify-rsa-4.0.1.tgz",
+      "resolved": "https://registry.npmjs.org/browserify-rsa/-/browserify-rsa-4.0.1.tgz",
       "integrity": "sha1-IeCr+vbyApzy+vsTNWenAdQTVSQ=",
       "dev": true,
       "requires": {
@@ -3427,7 +3438,7 @@
     },
     "camelcase-keys": {
       "version": "2.1.0",
-      "resolved": "http://registry.npmjs.org/camelcase-keys/-/camelcase-keys-2.1.0.tgz",
+      "resolved": "https://registry.npmjs.org/camelcase-keys/-/camelcase-keys-2.1.0.tgz",
       "integrity": "sha1-MIvur/3ygRkFHvodkyITyRuPkuc=",
       "dev": true,
       "requires": {
@@ -3630,7 +3641,7 @@
     "cipher-base": {
       "version": "1.0.4",
       "resolved": "https://registry.npmjs.org/cipher-base/-/cipher-base-1.0.4.tgz",
-      "integrity": "sha512-Kkht5ye6ZGmwv40uUDZztayT2ThLQGfnj/T71N/XzeZeo3nf8foyW7zGTsPYkEya3m5f3cAypH+qe7YOrM1U2Q==",
+      "integrity": "sha1-h2Dk7MJy9MNjUy+SbYdKriwTl94=",
       "dev": true,
       "requires": {
         "inherits": "^2.0.1",
@@ -3725,7 +3736,7 @@
         },
         "slice-ansi": {
           "version": "0.0.4",
-          "resolved": "http://registry.npmjs.org/slice-ansi/-/slice-ansi-0.0.4.tgz",
+          "resolved": "https://registry.npmjs.org/slice-ansi/-/slice-ansi-0.0.4.tgz",
           "integrity": "sha1-7b+JA/ZvfOL46v1s7tZeJkyDGzU=",
           "dev": true
         },
@@ -4321,7 +4332,7 @@
     },
     "create-hash": {
       "version": "1.2.0",
-      "resolved": "http://registry.npmjs.org/create-hash/-/create-hash-1.2.0.tgz",
+      "resolved": "https://registry.npmjs.org/create-hash/-/create-hash-1.2.0.tgz",
       "integrity": "sha512-z00bCGNHDG8mHAkP7CtT1qVu+bFQUPjYq/4Iv3C3kWjTFV10zIjfSoeqXo9Asws8gwSHDGj/hl2u4OGIjapeCg==",
       "dev": true,
       "requires": {
@@ -4334,7 +4345,7 @@
     },
     "create-hmac": {
       "version": "1.1.7",
-      "resolved": "http://registry.npmjs.org/create-hmac/-/create-hmac-1.1.7.tgz",
+      "resolved": "https://registry.npmjs.org/create-hmac/-/create-hmac-1.1.7.tgz",
       "integrity": "sha512-MJG9liiZ+ogc4TzUwuvbER1JRdgvUFSB5+VR/g5h82fGaIRWMWddtKBHi7/sVhfjQZ6SehlyhvQYrcYkaUIpLg==",
       "dev": true,
       "requires": {
@@ -4349,7 +4360,7 @@
     "createerror": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/createerror/-/createerror-1.3.0.tgz",
-      "integrity": "sha512-w9UZUtkaGd8MfS7eMG7Sa0lV5vCJghqQfiOnwNVrPhbZScUp5h0jwYoAF933MKlotlG1JAJOCCT3xU6r+SDKNw==",
+      "integrity": "sha1-xma9TNa5TjVBU5ZWnUZJ3QzbMxM=",
       "dev": true
     },
     "cross-env": {
@@ -4422,7 +4433,7 @@
     },
     "css-color-names": {
       "version": "0.0.4",
-      "resolved": "http://registry.npmjs.org/css-color-names/-/css-color-names-0.0.4.tgz",
+      "resolved": "https://registry.npmjs.org/css-color-names/-/css-color-names-0.0.4.tgz",
       "integrity": "sha1-gIrcLnnPhHOAabZGyyDsJ762KeA=",
       "dev": true
     },
@@ -5059,7 +5070,7 @@
     },
     "diffie-hellman": {
       "version": "5.0.3",
-      "resolved": "http://registry.npmjs.org/diffie-hellman/-/diffie-hellman-5.0.3.tgz",
+      "resolved": "https://registry.npmjs.org/diffie-hellman/-/diffie-hellman-5.0.3.tgz",
       "integrity": "sha512-kqag/Nl+f3GwyK25fhUMYj81BUOrZ9IuJsjIcDE5icNM9FJHAVm3VcUDxdLPoQtTuUylWm6ZIknYJwwaPxsUzg==",
       "dev": true,
       "requires": {
@@ -5269,7 +5280,7 @@
     },
     "duplexer": {
       "version": "0.1.1",
-      "resolved": "http://registry.npmjs.org/duplexer/-/duplexer-0.1.1.tgz",
+      "resolved": "https://registry.npmjs.org/duplexer/-/duplexer-0.1.1.tgz",
       "integrity": "sha1-rOb/gIwc5mtX0ev5eXessCM0z8E=",
       "dev": true
     },
@@ -5533,7 +5544,6 @@
       "version": "1.17.5",
       "resolved": "https://registry.npmjs.org/es-abstract/-/es-abstract-1.17.5.tgz",
       "integrity": "sha512-BR9auzDbySxOcfog0tLECW8l28eRGpDpU3Dm3Hp4q/N+VtLTmyj4EUN088XZWQDW/hzj6sYRDXeOFsaAODKvpg==",
-      "dev": true,
       "requires": {
         "es-to-primitive": "^1.2.1",
         "function-bind": "^1.1.1",
@@ -5548,11 +5558,36 @@
         "string.prototype.trimright": "^2.1.1"
       }
     },
+    "es-array-method-boxes-properly": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/es-array-method-boxes-properly/-/es-array-method-boxes-properly-1.0.0.tgz",
+      "integrity": "sha512-wd6JXUmyHmt8T5a2xreUwKcGPq6f1f+WwIJkijUqiGcJz1qqnZgP6XIK+QyIWU5lT7imeNxUll48bziG+TSYcA=="
+    },
+    "es-get-iterator": {
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/es-get-iterator/-/es-get-iterator-1.1.0.tgz",
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       "integrity": "sha512-QMEp5B7cftE7APOjk5Y6xgrbWu+WkLVQwk8JNjZ8nKRciZaByEW6MubieAiToS7+dwvrjGhH8jRXz3MVd0AYqQ==",
       "dev": true,
       "requires": {
@@ -15246,7 +15315,7 @@
     },
     "shasum": {
       "version": "1.0.2",
-      "resolved": "http://registry.npmjs.org/shasum/-/shasum-1.0.2.tgz",
+      "resolved": "https://registry.npmjs.org/shasum/-/shasum-1.0.2.tgz",
       "integrity": "sha1-5wEjENj0F/TetXEhUOVni4euVl8=",
       "dev": true,
       "requires": {
@@ -15851,7 +15920,7 @@
     },
     "split": {
       "version": "0.3.3",
-      "resolved": "http://registry.npmjs.org/split/-/split-0.3.3.tgz",
+      "resolved": "https://registry.npmjs.org/split/-/split-0.3.3.tgz",
       "integrity": "sha1-zQ7qXmOiEd//frDwkcQTPi0N0o8=",
       "dev": true,
       "requires": {
@@ -15950,7 +16019,7 @@
     },
     "starts-with": {
       "version": "1.0.2",
-      "resolved": "http://registry.npmjs.org/starts-with/-/starts-with-1.0.2.tgz",
+      "resolved": "https://registry.npmjs.org/starts-with/-/starts-with-1.0.2.tgz",
       "integrity": "sha1-Fnk6cp2J1M89T7LtovkIrjV/GW8=",
       "dev": true
     },
@@ -16005,7 +16074,7 @@
     },
     "stream-combiner": {
       "version": "0.0.4",
-      "resolved": "http://registry.npmjs.org/stream-combiner/-/stream-combiner-0.0.4.tgz",
+      "resolved": "https://registry.npmjs.org/stream-combiner/-/stream-combiner-0.0.4.tgz",
       "integrity": "sha1-TV5DPBhSYd3mI8o/RMWGvPXErRQ=",
       "dev": true,
       "requires": {
@@ -16152,7 +16221,6 @@
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/string.prototype.trimend/-/string.prototype.trimend-1.0.0.tgz",
       "integrity": "sha512-EEJnGqa/xNfIg05SxiPSqRS7S9qwDhYts1TSLR1BQfYUfPe1stofgGKvwERK9+9yf+PpfBMlpBaCHucXGPQfUA==",
-      "dev": true,
       "requires": {
         "define-properties": "^1.1.3",
         "es-abstract": "^1.17.5"
@@ -16162,7 +16230,6 @@
       "version": "2.1.2",
       "resolved": "https://registry.npmjs.org/string.prototype.trimleft/-/string.prototype.trimleft-2.1.2.tgz",
       "integrity": "sha512-gCA0tza1JBvqr3bfAIFJGqfdRTyPae82+KTnm3coDXkZN9wnuW3HjGgN386D7hfv5CHQYCI022/rJPVlqXyHSw==",
-      "dev": true,
       "requires": {
         "define-properties": "^1.1.3",
         "es-abstract": "^1.17.5",
@@ -16173,7 +16240,6 @@
       "version": "2.1.2",
       "resolved": "https://registry.npmjs.org/string.prototype.trimright/-/string.prototype.trimright-2.1.2.tgz",
       "integrity": "sha512-ZNRQ7sY3KroTaYjRS6EbNiiHrOkjihL9aQE/8gfQ4DtAC/aEBRHFJa44OmoWxGGqXuJlfKkZW4WcXErGr+9ZFg==",
-      "dev": true,
       "requires": {
         "define-properties": "^1.1.3",
         "es-abstract": "^1.17.5",
@@ -16184,7 +16250,6 @@
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/string.prototype.trimstart/-/string.prototype.trimstart-1.0.0.tgz",
       "integrity": "sha512-iCP8g01NFYiiBOnwG1Xc3WZLyoo+RuBymwIlWncShXDDJYWN6DbnM3odslBJdgCdRlq94B5s63NWAZlcn2CS4w==",
-      "dev": true,
       "requires": {
         "define-properties": "^1.1.3",
         "es-abstract": "^1.17.5"
@@ -16598,7 +16663,7 @@
       "dependencies": {
         "bluebird": {
           "version": "2.9.34",
-          "resolved": "http://registry.npmjs.org/bluebird/-/bluebird-2.9.34.tgz",
+          "resolved": "https://registry.npmjs.org/bluebird/-/bluebird-2.9.34.tgz",
           "integrity": "sha1-L3tOyAIWMoqf3evfacjUlC/v99g=",
           "dev": true
         },
@@ -16758,7 +16823,7 @@
     },
     "through": {
       "version": "2.3.8",
-      "resolved": "http://registry.npmjs.org/through/-/through-2.3.8.tgz",
+      "resolved": "https://registry.npmjs.org/through/-/through-2.3.8.tgz",
       "integrity": "sha1-DdTJ/6q8NXlgsbckEV1+Doai4fU=",
       "dev": true
     },
@@ -16839,7 +16904,7 @@
     "tmp": {
       "version": "0.0.33",
       "resolved": "https://registry.npmjs.org/tmp/-/tmp-0.0.33.tgz",
-      "integrity": "sha512-jRCJlojKnZ3addtTOjdIqoRuPEKBvNXcGYqzO6zWZX8KfKEpnGY5jfggJQ3EjKuu8D4bJRr0y+cYJFmYbImXGw==",
+      "integrity": "sha1-bTQzWIl2jSGyvNoKonfO07G/rfk=",
       "dev": true,
       "requires": {
         "os-tmpdir": "~1.0.2"
@@ -17182,9 +17247,9 @@
       "dev": true
     },
     "unexpected-eventemitter": {
-      "version": "1.1.3",
-      "resolved": "https://registry.npmjs.org/unexpected-eventemitter/-/unexpected-eventemitter-1.1.3.tgz",
-      "integrity": "sha512-30MfVuCOCSEvUzNUErYZ3ZzLiPOgADcJsyxi+0Z5bhwgI/Yv4xHR/2v/YEe2alaEXDdkteCQ4gLBfa5/J2iTPA==",
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/unexpected-eventemitter/-/unexpected-eventemitter-2.1.0.tgz",
+      "integrity": "sha512-qsXYpdVJzXhTLbxOwO9t7d0jabm5Wj6mzKFcbdsXI49Q6FxX0ntpCI3fh5rvgcpRH85AxHSQQp1uePcdw5MVDg==",
       "dev": true
     },
     "unexpected-sinon": {
@@ -18465,7 +18530,7 @@
     "webidl-conversions": {
       "version": "4.0.2",
       "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-4.0.2.tgz",
-      "integrity": "sha512-YQ+BmxuTgd6UXZW3+ICGfyqRyHXVlD5GtQr5+qjiNW7bF0cqrzX500HVXPBOvgXb5YnzDd+h0zqyv61KUD7+Sg==",
+      "integrity": "sha1-qFWYCx8LazWbodXZ+zmulB+qY60=",
       "dev": true
     },
     "whatwg-encoding": {
@@ -18573,6 +18638,11 @@
       "integrity": "sha1-J1hIEIkUVqQXHI0CJkQa3pDLyus=",
       "dev": true
     },
+    "workerpool": {
+      "version": "5.0.4",
+      "resolved": "https://registry.npmjs.org/workerpool/-/workerpool-5.0.4.tgz",
+      "integrity": "sha512-Sywova24Ow2NQ24JPB68bI89EdqMDjUXo4OpofK/QMD7C2ZVMloYBgQ5J3PChcBJHj2vspsmGx1/3nBKXtUkXQ=="
+    },
     "wrap-ansi": {
       "version": "5.1.0",
       "resolved": "https://registry.npmjs.org/wrap-ansi/-/wrap-ansi-5.1.0.tgz",
diff --git a/package-scripts.js b/package-scripts.js
index 7406018366..1270af93a7 100644
--- a/package-scripts.js
+++ b/package-scripts.js
@@ -10,10 +10,19 @@ const path = require('path');
  * @returns {string} Command string to be executed by nps.
  */
 function test(testName, mochaParams) {
-  const coverageCommand = `nyc --no-clean --report-dir coverage/reports/${testName}`;
+  let coverageCommand = `nyc --no-clean --report-dir coverage/reports/${testName}`;
   const mochaCommand = `node ${path.join('bin', 'mocha')}`; // Include 'node' and path.join for Windows compatibility
-  if (process.env.CI && !/^only-/.test(testName)) {
-    mochaParams += ' --forbid-only';
+  if (process.env.CI) {
+    // suppress coverage summaries in CI to reduce noise
+    coverageCommand += ' --reporter=json';
+    if (!/^only-/.test(testName)) {
+      mochaParams += ' --forbid-only';
+    }
+    mochaParams += ' --color';
+  }
+  // this may _actually_ be supported in the future
+  if (process.env.MOCHA_PARALLEL === '0') {
+    mochaParams += ' --no-parallel';
   }
   return `${
     process.env.COVERAGE ? coverageCommand : ''
@@ -23,7 +32,7 @@ function test(testName, mochaParams) {
 module.exports = {
   scripts: {
     build: {
-      script: `browserify -e browser-entry.js --plugin ./scripts/dedefine --ignore './lib/cli/*.js' --ignore "./lib/esm-utils.js" --ignore 'chokidar' --ignore 'fs' --ignore 'glob' --ignore 'path' --ignore 'supports-color' -o mocha.js`,
+      script: `browserify -e browser-entry.js --plugin ./scripts/dedefine --ignore './lib/cli/*.js' --ignore "./lib/esm-utils.js" --ignore 'chokidar' --ignore 'fs' --ignore 'glob' --ignore 'path' --ignore 'supports-color' --ignore './lib/buffered-runner.js' --ignore './lib/serializer.js' --ignore './lib/reporters/buffered.js' --ignore './lib/worker.js' -o mocha.js`,
       description: 'Build browser bundle'
     },
     lint: {
diff --git a/package.json b/package.json
index 777d26bd28..a1292cf476 100644
--- a/package.json
+++ b/package.json
@@ -66,10 +66,12 @@
     "minimatch": "3.0.4",
     "ms": "2.1.2",
     "object.assign": "4.1.0",
+    "promise.allsettled": "1.0.2",
     "strip-json-comments": "3.0.1",
     "supports-color": "7.1.0",
     "which": "2.0.2",
     "wide-align": "1.1.3",
+    "workerpool": "5.0.4",
     "yargs": "13.3.2",
     "yargs-parser": "13.1.2",
     "yargs-unparser": "1.6.0"
@@ -129,8 +131,8 @@
     "svgo": "^1.3.2",
     "through2": "^3.0.1",
     "to-vfile": "^6.1.0",
-    "unexpected": "^11.13.0",
-    "unexpected-eventemitter": "^1.1.3",
+    "unexpected": "^11.14.0",
+    "unexpected-eventemitter": "^2.1.0",
     "unexpected-sinon": "^10.11.2",
     "uslug": "^1.0.4",
     "watchify": "^3.11.1"
@@ -158,7 +160,11 @@
     "fs": false,
     "glob": false,
     "path": false,
-    "supports-color": false
+    "supports-color": false,
+    "./lib/serializer.js": false,
+    "./lib/reporters/buffered.js": false,
+    "./lib/buffered-reporter.js": false,
+    "./lib/worker.js": false
   },
   "prettier": {
     "singleQuote": true,
diff --git a/test/integration/fixtures/options/jobs/fail-in-parallel.fixture.js b/test/integration/fixtures/options/jobs/fail-in-parallel.fixture.js
new file mode 100644
index 0000000000..4601ed330c
--- /dev/null
+++ b/test/integration/fixtures/options/jobs/fail-in-parallel.fixture.js
@@ -0,0 +1,7 @@
+'use strict';
+
+it('should fail if in a worker', function() {
+  if (/worker\.js$/.test(require.main.filename)) {
+    throw new Error('in worker!');
+  }
+});
\ No newline at end of file
diff --git a/test/integration/fixtures/options/parallel/bail.fixture.js b/test/integration/fixtures/options/parallel/bail.fixture.js
new file mode 100644
index 0000000000..77692d56d1
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/bail.fixture.js
@@ -0,0 +1,9 @@
+describe('some suite', function() {
+  this.bail(true);
+  
+  it('should bail', function() {
+    throw new Error();
+  });
+
+  it('will not get run', function() {});
+});
diff --git a/test/integration/fixtures/options/parallel/retries-a.fixture.js b/test/integration/fixtures/options/parallel/retries-a.fixture.js
new file mode 100644
index 0000000000..e5d37ca561
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/retries-a.fixture.js
@@ -0,0 +1,5 @@
+describe('retry suite A', function() {
+  it('should pass', function() {
+
+  });
+});
\ No newline at end of file
diff --git a/test/integration/fixtures/options/parallel/retries-b.fixture.js b/test/integration/fixtures/options/parallel/retries-b.fixture.js
new file mode 100644
index 0000000000..c1f1d3f7ba
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/retries-b.fixture.js
@@ -0,0 +1,8 @@
+describe('retry suite B', function() {
+  let count = 0;
+  it('should retry', function() {
+    this.retries(3);
+    console.log(`count: ${++count}`);
+    throw new Error('failure');
+  });
+});
\ No newline at end of file
diff --git a/test/integration/fixtures/options/parallel/syntax-err.fixture.js b/test/integration/fixtures/options/parallel/syntax-err.fixture.js
new file mode 100644
index 0000000000..8dd2b0f93b
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/syntax-err.fixture.js
@@ -0,0 +1 @@
+var foo = 
\ No newline at end of file
diff --git a/test/integration/fixtures/options/parallel/test-a.fixture.js b/test/integration/fixtures/options/parallel/test-a.fixture.js
new file mode 100644
index 0000000000..43f53bbda8
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/test-a.fixture.js
@@ -0,0 +1,3 @@
+describe('a', function() {
+  it('should pass', function() {});
+});
diff --git a/test/integration/fixtures/options/parallel/test-b.fixture.js b/test/integration/fixtures/options/parallel/test-b.fixture.js
new file mode 100644
index 0000000000..8e6437a56a
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/test-b.fixture.js
@@ -0,0 +1,3 @@
+describe('b', function() {
+  it('should be pending');
+});
diff --git a/test/integration/fixtures/options/parallel/test-c.fixture.js b/test/integration/fixtures/options/parallel/test-c.fixture.js
new file mode 100644
index 0000000000..d06b6a3ee6
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/test-c.fixture.js
@@ -0,0 +1,5 @@
+describe('c', function() {
+  it('should fail', function() {
+    throw new Error('failure');
+  });
+});
diff --git a/test/integration/fixtures/options/parallel/test-d.fixture.js b/test/integration/fixtures/options/parallel/test-d.fixture.js
new file mode 100644
index 0000000000..ee19d54594
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/test-d.fixture.js
@@ -0,0 +1,7 @@
+describe('d', function() {
+  it('should pass, then fail', function() {
+    process.nextTick(function() {
+      throw new Error('uncaught!!');
+    });
+  });
+});
diff --git a/test/integration/fixtures/options/parallel/uncaught.fixture.js b/test/integration/fixtures/options/parallel/uncaught.fixture.js
new file mode 100644
index 0000000000..bca1610ab7
--- /dev/null
+++ b/test/integration/fixtures/options/parallel/uncaught.fixture.js
@@ -0,0 +1,7 @@
+'use strict';
+
+it('throws an uncaught exception', function (done) {
+  process.nextTick(function () {
+    throw new Error('existential isolation!!');
+  });
+});
diff --git a/test/integration/helpers.js b/test/integration/helpers.js
index 17a1acfea1..35995fa2f7 100644
--- a/test/integration/helpers.js
+++ b/test/integration/helpers.js
@@ -241,11 +241,29 @@ function toJSONRunResult(result) {
 /**
  * Creates arguments loading a default fixture if none provided
  *
+ * - The `--no-color` arg is always used (color output complicates testing `STDOUT`)
+ * - Unless `--bail` or `--no-bail` is set, use `--no-bail`.  This enables using
+ *   `--bail` (if desired) from the command-line when running our integration
+ *   test suites without stepping on the toes of subprocesses.
+ * - Unless `--parallel` or `--no-parallel` is set, use `--no-parallel`.  We
+ *   assume the test suite is _already_ running in parallel--and there's no point
+ *   in trying to run a single test fixture in parallel.
+ * - The {@link DEFAULT_FIXTURE} file is used if no arguments are provided.
+ *
  * @param {string[]|*} [args] - Arguments to `spawn`
  * @returns string[]
  */
 function defaultArgs(args) {
-  return !args || !args.length ? ['--file', DEFAULT_FIXTURE] : args;
+  var newArgs = (!args || !args.length ? [DEFAULT_FIXTURE] : args).concat([
+    '--no-color'
+  ]);
+  if (!newArgs.some(arg => /--(no-)?bail/.test(arg))) {
+    newArgs.push('--no-bail');
+  }
+  if (!newArgs.some(arg => /--(no-)?parallel/.test(arg))) {
+    newArgs.push('--no-parallel');
+  }
+  return newArgs;
 }
 
 function invokeMocha(args, fn, opts) {
diff --git a/test/integration/options/jobs.spec.js b/test/integration/options/jobs.spec.js
new file mode 100644
index 0000000000..832d825298
--- /dev/null
+++ b/test/integration/options/jobs.spec.js
@@ -0,0 +1,34 @@
+'use strict';
+
+var helpers = require('../helpers');
+var runMochaAsync = helpers.runMochaAsync;
+
+describe('--jobs', function() {
+  describe('when set to a number less than 2', function() {
+    it('should run tests in serial', function() {
+      return expect(
+        runMochaAsync(
+          'options/jobs/fail-in-parallel',
+          ['--parallel', '--jobs', '1'],
+          'pipe'
+        ),
+        'when fulfilled',
+        'to have passed'
+      );
+    });
+  });
+
+  describe('when set to a number greater than 1', function() {
+    it('should run tests in parallel', function() {
+      return expect(
+        runMochaAsync(
+          'options/jobs/fail-in-parallel',
+          ['--parallel', '--jobs', '2'],
+          'pipe'
+        ),
+        'when fulfilled',
+        'to have failed'
+      );
+    });
+  });
+});
diff --git a/test/integration/options/parallel.spec.js b/test/integration/options/parallel.spec.js
new file mode 100644
index 0000000000..d860e35429
--- /dev/null
+++ b/test/integration/options/parallel.spec.js
@@ -0,0 +1,442 @@
+'use strict';
+var Mocha = require('../../../lib/mocha');
+var path = require('path');
+var helpers = require('../helpers');
+var runMochaAsync = helpers.runMochaAsync;
+var invokeMochaAsync = helpers.invokeMochaAsync;
+var getSummary = helpers.getSummary;
+var utils = require('../../../lib/utils');
+
+function compareReporters(reporter) {
+  this.timeout(Math.max(this.timeout(), 5000));
+  return runMochaAsync(path.join('options', 'parallel', 'test-a.fixture.js'), [
+    '--reporter',
+    reporter,
+    '--no-parallel'
+  ]).then(function(expected) {
+    expected.output = expected.output.replace(/\d+ms/g, /100ms/);
+    return runMochaAsync(
+      path.join('options', 'parallel', 'test-a.fixture.js'),
+      ['--reporter', reporter, '--parallel']
+    ).then(function(actual) {
+      actual.output = actual.output.replace(/\d+ms/g, /100ms/);
+      return [actual, expected];
+    });
+  });
+}
+
+function runGenericReporterTest(reporter) {
+  return compareReporters.call(this, reporter).then(function(result) {
+    var expected = result.shift();
+    var actual = result.shift();
+    return expect(actual, 'to satisfy', {
+      passing: expected.passing,
+      failing: expected.failing,
+      pending: expected.pending,
+      code: expected.code,
+      output: expected.output
+    });
+  });
+}
+
+describe('--parallel', function() {
+  describe('when a test has a syntax error', function() {
+    describe('when there is only a single test file', function() {
+      it('should fail gracefully', function() {
+        return expect(
+          runMochaAsync('options/parallel/syntax-err', ['--parallel']),
+          'when fulfilled',
+          'to have failed with output',
+          /SyntaxError/
+        );
+      });
+    });
+
+    describe('when there are multiple test files', function() {
+      it('should fail gracefully', function() {
+        return expect(
+          invokeMochaAsync(
+            [
+              require.resolve(
+                '../fixtures/options/parallel/syntax-err.fixture.js'
+              ),
+              '--parallel'
+            ],
+            'pipe'
+          )[1],
+          'when fulfilled',
+          'to have failed'
+        );
+      });
+    });
+  });
+
+  describe('when used with CJS tests', function() {
+    it('should have the same result as with --no-parallel', function() {
+      this.timeout(Math.max(this.timeout(), 5000));
+      return runMochaAsync(
+        path.join('options', 'parallel', 'test-*.fixture.js'),
+        ['--no-parallel']
+      ).then(function(expected) {
+        return expect(
+          runMochaAsync(path.join('options', 'parallel', 'test-*.fixture.js'), [
+            '--parallel'
+          ]),
+          'to be fulfilled with value satisfying',
+          {
+            passing: expected.passing,
+            failing: expected.failing,
+            pending: expected.pending,
+            code: expected.code
+          }
+        );
+      });
+    });
+  });
+
+  describe('when used with ESM tests', function() {
+    var esmArgs =
+      Number(process.versions.node.split('.')[0]) >= 13
+        ? []
+        : ['--experimental-modules'];
+
+    before(function() {
+      if (!utils.supportsEsModules()) this.skip();
+    });
+
+    it('should have the same result as with --no-parallel', function() {
+      this.timeout(Math.min(this.timeout(), 5000));
+
+      var args = [
+        path.join(__dirname, '..', 'fixtures', 'esm', '*.fixture.mjs')
+      ].concat(esmArgs);
+      return invokeMochaAsync(args.concat('--no-parallel'))[1].then(function(
+        expected
+      ) {
+        var expectedSummary = getSummary(expected);
+        return invokeMochaAsync(args.concat('--parallel'))[1].then(function(
+          actual
+        ) {
+          var actualSummary = getSummary(actual);
+          expect(actualSummary, 'to satisfy', {
+            pending: expectedSummary.pending,
+            passing: expectedSummary.passing,
+            failing: expectedSummary.failing
+          });
+        });
+      });
+    });
+  });
+
+  describe('when used with --retries', function() {
+    it('should retry tests appropriately', function() {
+      return expect(
+        runMochaAsync(
+          path.join('options', 'parallel', 'retries-*.fixture.js'),
+          ['--parallel']
+        ),
+        'when fulfilled',
+        'to have failed'
+      )
+        .and('when fulfilled', 'to have passed test count', 1)
+        .and('when fulfilled', 'to have pending test count', 0)
+        .and('when fulfilled', 'to have failed test count', 1)
+        .and('when fulfilled', 'to contain output', /count: 3/);
+    });
+  });
+
+  describe('when used with --allow-uncaught', function() {
+    it('should bubble up an exception', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            require.resolve('../fixtures/options/parallel/uncaught.fixture.js'),
+            '--parallel',
+            '--allow-uncaught'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to contain output',
+        /Error: existential isolation/i
+      ).and('when fulfilled', 'to have exit code', 1);
+    });
+  });
+
+  describe('when used with --file', function() {
+    it('should error out', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            '--file',
+            path.join('options', 'parallel', 'test-a.fixture.js'),
+            '--parallel'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to contain output',
+        /mutually exclusive with --file/
+      );
+    });
+  });
+
+  describe('when used with --sort', function() {
+    it('should error out', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            '--sort',
+            path.join('options', 'parallel', 'test-*.fixture.js'),
+            '--parallel'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to contain output',
+        /mutually exclusive with --sort/
+      );
+    });
+  });
+
+  describe('when used with --bail', function() {
+    it('should skip some tests', function() {
+      return runMochaAsync(
+        path.join('options', 'parallel', 'test-*.fixture.js'),
+        ['--parallel', '--bail']
+      ).then(function(result) {
+        // we don't know _exactly_ how many tests will be skipped here
+        // due to the --bail, but the number of tests completed should be
+        // less than the total, which is 5.
+        return expect(
+          result.passing + result.pending + result.failing,
+          'to be less than',
+          5
+        );
+      });
+    });
+
+    it('should fail', function() {
+      return expect(
+        runMochaAsync(path.join('options', 'parallel', 'test-*.fixture.js'), [
+          '--parallel',
+          '--bail'
+        ]),
+        'when fulfilled',
+        'to have failed'
+      );
+    });
+  });
+
+  describe('when encountering a "bail" in context', function() {
+    it('should skip some tests', function() {
+      return runMochaAsync('options/parallel/bail', ['--parallel']).then(
+        function(result) {
+          return expect(
+            result.passing + result.pending + result.failing,
+            'to be less than',
+            2
+          );
+        }
+      );
+    });
+
+    it('should fail', function() {
+      return expect(
+        runMochaAsync('options/parallel/bail', ['--parallel', '--bail']),
+        'when fulfilled',
+        'to have failed'
+      );
+    });
+  });
+
+  describe('reporter equivalence', function() {
+    // each reporter name is duplicated; one is in all lower-case
+    // 'base' is abstract, 'html' is browser-only, others are incompatible
+    var DENY = ['progress', 'base', 'html', 'markdown', 'json-stream'];
+    Object.keys(Mocha.reporters)
+      .filter(function(name) {
+        return /^[a-z]/.test(name) && DENY.indexOf(name) === -1;
+      })
+      .forEach(function(reporter) {
+        describe(
+          'when multiple test files run with --reporter=' + reporter,
+          function() {
+            it('should have the same result as when run with --no-parallel', function() {
+              // note that the output may not be in the same order, as running file
+              // order is non-deterministic in parallel mode
+              this.timeout(Math.max(this.timeout(), 5000));
+              return runMochaAsync(
+                path.join('options', 'parallel', 'test-*.fixture.js'),
+                ['--reporter', reporter, '--no-parallel']
+              ).then(function(expected) {
+                return expect(
+                  runMochaAsync(
+                    path.join('options', 'parallel', 'test-*.fixture.js'),
+                    ['--reporter', reporter, '--parallel']
+                  ),
+                  'to be fulfilled with value satisfying',
+                  {
+                    passing: expected.passing,
+                    failing: expected.failing,
+                    pending: expected.pending,
+                    code: expected.code
+                  }
+                );
+              });
+            });
+          }
+        );
+      });
+  });
+
+  describe('when a single test file is run with --reporter=dot', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'dot');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=doc', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'doc');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=tap', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'tap');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=list', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'list');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=min', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'min');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=spec', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'spec');
+    });
+  });
+
+  describe('when used with --reporter=nyan', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'nyan');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=landing', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      return runGenericReporterTest.call(this, 'landing');
+    });
+  });
+
+  describe('when a single test file is run with --reporter=progress', function() {
+    it('should fail due to incompatibility', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            require.resolve('../fixtures/options/parallel/test-a.fixture.js'),
+            '--reporter=progress',
+            '--parallel'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to have failed'
+      ).and('when fulfilled', 'to contain output', /mutually exclusive/);
+    });
+  });
+
+  describe('when a single test file is run with --reporter=markdown', function() {
+    it('should fail due to incompatibility', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            require.resolve('../fixtures/options/parallel/test-a.fixture.js'),
+            '--reporter=markdown',
+            '--parallel'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to have failed'
+      ).and('when fulfilled', 'to contain output', /mutually exclusive/);
+    });
+  });
+
+  describe('when a single test file is run with --reporter=json-stream', function() {
+    it('should fail due to incompatibility', function() {
+      return expect(
+        invokeMochaAsync(
+          [
+            require.resolve('../fixtures/options/parallel/test-a.fixture.js'),
+            '--reporter=json-stream',
+            '--parallel'
+          ],
+          'pipe'
+        )[1],
+        'when fulfilled',
+        'to have failed'
+      ).and('when fulfilled', 'to contain output', /mutually exclusive/);
+    });
+  });
+
+  describe('when a single test file is run with --reporter=json', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      // this one has some timings/durations that we can safely ignore
+      return compareReporters.call(this, 'json').then(function(result) {
+        var expected = result.shift();
+        expected.output = JSON.parse(expected.output);
+        var actual = result.shift();
+        actual.output = JSON.parse(actual.output);
+        return expect(actual, 'to satisfy', {
+          passing: expected.passing,
+          failing: expected.failing,
+          pending: expected.pending,
+          code: expected.code,
+          output: {
+            stats: {
+              suites: expected.output.stats.suites,
+              tests: expected.output.stats.tests,
+              passes: expected.output.stats.passes,
+              pending: expected.output.stats.pending,
+              failures: expected.output.stats.failures
+            },
+            tests: expected.tests
+          }
+        });
+      });
+    });
+  });
+
+  describe('when a single test file is run with --reporter=xunit', function() {
+    it('should have the same output as when run with --no-parallel', function() {
+      // durations need replacing
+      return compareReporters.call(this, 'xunit').then(function(result) {
+        var expected = result.shift();
+        expected.output = expected.output
+          .replace(/time=".+?"/g, 'time="0.5"')
+          .replace(/timestamp=".+?"/g, 'timestamp="some-timestamp');
+        var actual = result.shift();
+        actual.output = actual.output
+          .replace(/time=".+?"/g, 'time="0.5"')
+          .replace(/timestamp=".+?"/g, 'timestamp="some-timestamp');
+        return expect(actual, 'to satisfy', {
+          passing: expected.passing,
+          failing: expected.failing,
+          pending: expected.pending,
+          code: expected.code,
+          output: expected.output
+        });
+      });
+    });
+  });
+});
diff --git a/test/node-unit/buffered-runner.spec.js b/test/node-unit/buffered-runner.spec.js
new file mode 100644
index 0000000000..0330c62241
--- /dev/null
+++ b/test/node-unit/buffered-runner.spec.js
@@ -0,0 +1,596 @@
+'use strict';
+
+const {
+  EVENT_RUN_BEGIN,
+  EVENT_TEST_PASS,
+  EVENT_TEST_FAIL,
+  EVENT_SUITE_END,
+  EVENT_SUITE_BEGIN
+} = require('../../lib/runner').constants;
+const rewiremock = require('rewiremock/node');
+const BUFFERED_RUNNER_PATH = require.resolve('../../lib/buffered-runner.js');
+const Suite = require('../../lib/suite');
+const {createSandbox} = require('sinon');
+
+describe('buffered-runner', function() {
+  describe('BufferedRunner', function() {
+    let sandbox;
+    let pool;
+    let run;
+    let terminate;
+    let BufferedRunner;
+    let suite;
+    let warn;
+    let cpuCount;
+
+    beforeEach(function() {
+      sandbox = createSandbox();
+      cpuCount = 1;
+      suite = new Suite('a root suite', {}, true);
+      warn = sandbox.stub();
+
+      // tests will want to further define the behavior of these.
+      run = sandbox.stub();
+      terminate = sandbox.stub();
+
+      pool = sandbox.stub().returns({
+        proxy: sandbox.stub().resolves({
+          run
+        }),
+        terminate,
+        stats: sandbox.stub().returns({})
+      });
+      BufferedRunner = rewiremock.proxy(BUFFERED_RUNNER_PATH, r => ({
+        workerpool: {
+          pool
+        },
+        os: {
+          cpus: sandbox.stub().callsFake(() => new Array(cpuCount))
+        },
+        [require.resolve('../../lib/utils')]: r.with({warn}).callThrough()
+      }));
+    });
+
+    describe('instance method', function() {
+      describe('run', function() {
+        let runner;
+
+        beforeEach(function() {
+          runner = new BufferedRunner(suite);
+        });
+
+        // the purpose of this is to ensure that--despite using `Promise`s
+        // internally--`BufferedRunner#run` does not return a `Promise`.
+        it('should be chainable', function(done) {
+          expect(runner.run(done, {files: [], options: {}}), 'to be', runner);
+        });
+
+        it('should emit `EVENT_RUN_BEGIN`', async function() {
+          return expect(
+            () =>
+              new Promise(resolve => {
+                runner.run(resolve, {files: [], options: {}});
+              }),
+            'to emit from',
+            runner,
+            EVENT_RUN_BEGIN
+          );
+        });
+
+        describe('when a worker fails', function() {
+          it('should recover', function(done) {
+            const options = {};
+            run.withArgs('some-file.js', options).rejects(new Error('whoops'));
+            run.withArgs('some-other-file.js', options).resolves({
+              failureCount: 0,
+              events: [
+                {
+                  eventName: EVENT_TEST_PASS,
+                  data: {
+                    title: 'some test'
+                  }
+                },
+                {
+                  eventName: EVENT_SUITE_END,
+                  data: {
+                    title: 'some suite'
+                  }
+                }
+              ]
+            });
+
+            runner.run(
+              () => {
+                expect(terminate, 'to have calls satisfying', [{args: []}]);
+                done();
+              },
+              {
+                files: ['some-file.js', 'some-other-file.js'],
+                options
+              }
+            );
+          });
+
+          it('should delegate to Runner#uncaught', function(done) {
+            const options = {};
+            sandbox.spy(runner, 'uncaught');
+            const err = new Error('whoops');
+            run.withArgs('some-file.js', options).rejects(new Error('whoops'));
+            run.withArgs('some-other-file.js', options).resolves({
+              failureCount: 0,
+              events: [
+                {
+                  eventName: EVENT_TEST_PASS,
+                  data: {
+                    title: 'some test'
+                  }
+                },
+                {
+                  eventName: EVENT_SUITE_END,
+                  data: {
+                    title: 'some suite'
+                  }
+                }
+              ]
+            });
+
+            runner.run(
+              () => {
+                expect(runner.uncaught, 'to have a call satisfying', [err]);
+                done();
+              },
+              {
+                files: ['some-file.js', 'some-other-file.js'],
+                options
+              }
+            );
+          });
+        });
+
+        describe('when not provided a max job count', function() {
+          it('should use a max job count based on CPU cores', function(done) {
+            runner.run(
+              () => {
+                expect(pool, 'to have a call satisfying', {
+                  args: [
+                    expect.it('to be a', 'string'),
+                    {
+                      maxWorkers: Math.max(cpuCount - 1, 1)
+                    }
+                  ]
+                });
+                done();
+              },
+              {files: [], options: {}}
+            );
+          });
+        });
+
+        describe('when provided a max job count', function() {
+          beforeEach(function() {
+            cpuCount = 8;
+          });
+
+          it('should use the provided max count', function(done) {
+            runner.run(
+              () => {
+                expect(pool, 'to have a call satisfying', {
+                  args: [
+                    expect.it('to be a', 'string'),
+                    {
+                      maxWorkers: 4
+                    }
+                  ]
+                });
+                done();
+              },
+              {
+                files: [],
+                options: {
+                  jobs: 4
+                }
+              }
+            );
+          });
+
+          describe('when the max job count exceeds the CPU count', function() {
+            it('should warn', function(done) {
+              run.resolves({failureCount: 0, events: []});
+              runner.run(
+                () => {
+                  expect(warn, 'to have a call satisfying', [
+                    /only enough cores available/
+                  ]);
+                  done();
+                },
+                {
+                  files: [],
+                  options: {jobs: 16}
+                }
+              );
+            });
+          });
+
+          describe('when there are not enough CPU cores', function() {
+            beforeEach(function() {
+              cpuCount = 2;
+            });
+
+            it('should warn', function(done) {
+              run.resolves({failureCount: 0, events: []});
+              runner.run(
+                () => {
+                  expect(warn, 'to have a call satisfying', [
+                    /avoid --parallel on this machine/
+                  ]);
+                  done();
+                },
+                {
+                  files: [],
+                  options: {jobs: 4}
+                }
+              );
+            });
+          });
+        });
+
+        describe('when suite should bail', function() {
+          describe('when no event contains an error', function() {
+            it('should not force-terminate', function(done) {
+              run.resolves({
+                failureCount: 0,
+                events: [
+                  {
+                    eventName: EVENT_SUITE_BEGIN,
+                    data: {
+                      title: 'some suite',
+                      _bail: true
+                    }
+                  },
+                  {
+                    eventName: EVENT_TEST_PASS,
+                    data: {
+                      title: 'some test'
+                    }
+                  },
+                  {
+                    eventName: EVENT_SUITE_END,
+                    data: {
+                      title: 'some suite',
+                      _bail: true
+                    }
+                  }
+                ]
+              });
+
+              runner.run(
+                () => {
+                  expect(terminate, 'to have a call satisfying', {
+                    args: []
+                  }).and('was called once');
+                  done();
+                },
+                {
+                  files: ['some-file.js', 'some-other-file.js'],
+                  options: {}
+                }
+              );
+            });
+          });
+
+          describe('when an event contains an error and has positive failures', function() {
+            describe('when subsequent files have not yet been run', function() {
+              it('should cleanly terminate the thread pool', function(done) {
+                const options = {};
+                const err = {
+                  __type: 'Error',
+                  message: 'oh no'
+                };
+                run.withArgs('some-file.js', options).resolves({
+                  failureCount: 1,
+                  events: [
+                    {
+                      eventName: EVENT_SUITE_BEGIN,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    },
+                    {
+                      eventName: EVENT_TEST_FAIL,
+                      data: {
+                        title: 'some test'
+                      },
+                      error: err
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    }
+                  ]
+                });
+                run.withArgs('some-other-file.js', options).rejects();
+
+                runner.run(
+                  () => {
+                    expect(terminate, 'to have calls satisfying', [
+                      {args: []}, // this is the pool force-terminating
+                      {args: []} // this will always be called, and will do nothing due to the previous call
+                    ]).and('was called twice');
+                    done();
+                  },
+                  {
+                    files: ['some-file.js', 'some-other-file.js'],
+                    options
+                  }
+                );
+              });
+            });
+            describe('when subsequent files already started running', function() {
+              it('should cleanly terminate the thread pool', function(done) {
+                const options = {};
+                const err = {
+                  __type: 'Error',
+                  message: 'oh no'
+                };
+                run.withArgs('some-file.js', options).resolves({
+                  failureCount: 1,
+                  events: [
+                    {
+                      eventName: EVENT_SUITE_BEGIN,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    },
+                    {
+                      eventName: EVENT_TEST_FAIL,
+                      data: {
+                        title: 'some test'
+                      },
+                      error: err
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    }
+                  ]
+                });
+                run.withArgs('some-other-file.js', options).resolves({
+                  failureCount: 0,
+                  events: [
+                    {
+                      eventName: EVENT_SUITE_BEGIN,
+                      data: {
+                        title: 'some suite'
+                      }
+                    },
+                    {
+                      eventName: EVENT_TEST_PASS,
+                      data: {
+                        title: 'some test'
+                      }
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite'
+                      }
+                    }
+                  ]
+                });
+
+                runner.run(
+                  () => {
+                    expect(terminate, 'to have calls satisfying', [
+                      {args: []}, // this is the pool force-terminating
+                      {args: []} // this will always be called, and will do nothing due to the previous call
+                    ]).and('was called twice');
+                    done();
+                  },
+                  {
+                    files: ['some-file.js', 'some-other-file.js'],
+                    options
+                  }
+                );
+              });
+            });
+          });
+        });
+
+        describe('when a suite has a bail flag', function() {
+          describe('when no event contains an error', function() {
+            it('should not force-terminate', function(done) {
+              run.resolves({
+                failureCount: 0,
+                events: [
+                  {
+                    eventName: EVENT_TEST_PASS,
+                    data: {
+                      title: 'some test'
+                    }
+                  },
+                  {
+                    eventName: EVENT_SUITE_END,
+                    data: {
+                      title: 'some suite',
+                      _bail: true
+                    }
+                  }
+                ]
+              });
+
+              runner.run(
+                () => {
+                  expect(terminate, 'to have a call satisfying', {
+                    args: []
+                  }).and('was called once');
+                  done();
+                },
+                {
+                  files: ['some-file.js', 'some-other-file.js'],
+                  options: {}
+                }
+              );
+            });
+          });
+          describe('when an event contains an error and has positive failures', function() {
+            describe('when subsequent files have not yet been run', function() {
+              it('should cleanly terminate the thread pool', function(done) {
+                const options = {};
+                const err = {
+                  __type: 'Error',
+                  message: 'oh no'
+                };
+                run.withArgs('some-file.js', options).resolves({
+                  failureCount: 1,
+                  events: [
+                    {
+                      eventName: EVENT_TEST_FAIL,
+                      data: {
+                        title: 'some test'
+                      },
+                      error: err
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    }
+                  ]
+                });
+                run.withArgs('some-other-file.js', options).rejects();
+
+                runner.run(
+                  () => {
+                    expect(terminate, 'to have calls satisfying', [
+                      {args: []}, // this is the pool force-terminating
+                      {args: []} // this will always be called, and will do nothing due to the previous call
+                    ]).and('was called twice');
+                    done();
+                  },
+                  {
+                    files: ['some-file.js', 'some-other-file.js'],
+                    options
+                  }
+                );
+              });
+            });
+
+            describe('when subsequent files already started running', function() {
+              it('should cleanly terminate the thread pool', function(done) {
+                const options = {};
+                const err = {
+                  __type: 'Error',
+                  message: 'oh no'
+                };
+                run.withArgs('some-file.js', options).resolves({
+                  failureCount: 1,
+                  events: [
+                    {
+                      eventName: EVENT_TEST_FAIL,
+                      data: {
+                        title: 'some test'
+                      },
+                      error: err
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    }
+                  ]
+                });
+                run.withArgs('some-other-file.js', options).resolves({
+                  failureCount: 0,
+                  events: [
+                    {
+                      eventName: EVENT_TEST_PASS,
+                      data: {
+                        title: 'some test'
+                      }
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite'
+                      }
+                    }
+                  ]
+                });
+
+                runner.run(
+                  () => {
+                    expect(terminate, 'to have calls satisfying', [
+                      {args: []}, // this is the pool force-terminating
+                      {args: []} // this will always be called, and will do nothing due to the previous call
+                    ]).and('was called twice');
+                    done();
+                  },
+                  {
+                    files: ['some-file.js', 'some-other-file.js'],
+                    options
+                  }
+                );
+              });
+            });
+
+            describe('when subsequent files have not yet been run', function() {
+              it('should cleanly terminate the thread pool', function(done) {
+                const options = {};
+                const err = {
+                  __type: 'Error',
+                  message: 'oh no'
+                };
+                run.withArgs('some-file.js', options).resolves({
+                  failureCount: 1,
+                  events: [
+                    {
+                      eventName: EVENT_TEST_FAIL,
+                      data: {
+                        title: 'some test'
+                      },
+                      error: err
+                    },
+                    {
+                      eventName: EVENT_SUITE_END,
+                      data: {
+                        title: 'some suite',
+                        _bail: true
+                      }
+                    }
+                  ]
+                });
+                run.withArgs('some-other-file.js', options).rejects();
+
+                runner.run(
+                  () => {
+                    expect(terminate, 'to have calls satisfying', [
+                      {args: []}, // this is the pool force-terminating
+                      {args: []} // this will always be called, and will do nothing due to the previous call
+                    ]).and('was called twice');
+                    done();
+                  },
+                  {
+                    files: ['some-file.js', 'some-other-file.js'],
+                    options
+                  }
+                );
+              });
+            });
+          });
+        });
+      });
+    });
+  });
+});
diff --git a/test/node-unit/serializer.spec.js b/test/node-unit/serializer.spec.js
new file mode 100644
index 0000000000..437f133a4e
--- /dev/null
+++ b/test/node-unit/serializer.spec.js
@@ -0,0 +1,555 @@
+'use strict';
+
+const {createSandbox} = require('sinon');
+const {
+  serialize,
+  deserialize,
+  SerializableEvent,
+  SerializableWorkerResult
+} = require('../../lib/serializer');
+
+describe('serializer', function() {
+  let sandbox;
+
+  beforeEach(function() {
+    sandbox = createSandbox();
+  });
+
+  afterEach(function() {
+    sandbox.restore();
+  });
+
+  describe('function', function() {
+    describe('serialize', function() {
+      describe('when passed a non-object value', function() {
+        it('should return the value', function() {
+          expect(serialize('knees & toes'), 'to be', 'knees & toes');
+        });
+      });
+
+      describe('when passed an object value', function() {
+        describe('w/o a `serialize` method', function() {
+          it('should return the value', function() {
+            const obj = {};
+            expect(serialize(obj), 'to be', obj);
+          });
+        });
+
+        describe('having a `serialize` method', function() {
+          it('should return the result of the `serialize` method', function() {
+            const serializedObj = {foo: 'bar'};
+            const obj = {serialize: sandbox.stub().returns(serializedObj)};
+            expect(serialize(obj), 'to be', serializedObj);
+          });
+        });
+      });
+
+      describe('when not passed anything', function() {
+        it('should return `undefined`', function() {
+          expect(serialize(), 'to be undefined');
+        });
+      });
+    });
+
+    describe('deserialize', function() {
+      describe('when passed nothing', function() {
+        it('should return `undefined`', function() {
+          expect(deserialize(), 'to be undefined');
+        });
+      });
+
+      describe('when passed a non-object value', function() {
+        it('should return the value', function() {
+          expect(deserialize(500), 'to be', 500);
+        });
+      });
+
+      describe('when passed an object value which is not a SerializedWorkerResult', function() {
+        it('should return the value', function() {
+          const obj = {};
+          expect(deserialize(obj), 'to be', obj);
+        });
+      });
+
+      describe('when passed a SerializedWorkerResult object', function() {
+        // note that SerializedWorkerResult is an interface (typedef), not a class.
+
+        it('should return the result of `SerializableWorkerResult.deserialize` called on the value', function() {
+          const obj = Object.assign({}, SerializableWorkerResult.create());
+          sandbox
+            .stub(SerializableWorkerResult, 'deserialize')
+            .returns('butts');
+          deserialize(obj);
+          expect(
+            SerializableWorkerResult.deserialize,
+            'to have a call satisfying',
+            {
+              args: [obj],
+              returned: 'butts'
+            }
+          );
+        });
+      });
+    });
+  });
+
+  describe('SerializableEvent', function() {
+    describe('constructor', function() {
+      describe('when called without `eventName`', function() {
+        it('should throw', function() {
+          expect(
+            () => new SerializableEvent(),
+            'to throw',
+            /expected a non-empty `eventName`/
+          );
+        });
+      });
+
+      describe('when called with a non-object `rawObject`', function() {
+        it('should throw', function() {
+          expect(
+            () => new SerializableEvent('blub', 'glug'),
+            'to throw',
+            /expected object, received \[string\]/
+          );
+        });
+      });
+    });
+
+    describe('instance method', function() {
+      describe('serialize', function() {
+        it('should mutate the instance in-place', function() {
+          const evt = SerializableEvent.create('foo');
+          expect(evt.serialize(), 'to be', evt);
+        });
+
+        it('should freeze the instance', function() {
+          expect(
+            Object.isFrozen(SerializableEvent.create('foo').serialize()),
+            'to be true'
+          );
+        });
+
+        describe('when passed an object with a `serialize` method', function() {
+          it('should call the `serialize` method', function() {
+            const obj = {
+              serialize: sandbox.stub()
+            };
+            SerializableEvent.create('some-event', obj).serialize();
+            expect(obj.serialize, 'was called once');
+          });
+        });
+
+        describe('when passed an object containing an object with a `serialize` method', function() {
+          it('should call the `serialize` method', function() {
+            const stub = sandbox.stub();
+            const obj = {
+              nested: {
+                serialize: stub
+              }
+            };
+            SerializableEvent.create('some-event', obj).serialize();
+            expect(stub, 'was called once');
+          });
+        });
+
+        describe('when passed an object containing a non-`serialize` method', function() {
+          it('should remove the method', function() {
+            const obj = {
+              func: () => {}
+            };
+
+            expect(
+              SerializableEvent.create('some-event', obj).serialize(),
+              'to satisfy',
+              {
+                data: expect.it('not to have property', 'func')
+              }
+            );
+          });
+        });
+
+        describe('when passed an object containing an array', function() {
+          it('should serialize the array', function() {
+            const obj = {
+              list: [{herp: 'derp'}, {bing: 'bong'}]
+            };
+            expect(
+              SerializableEvent.create('some-event', obj).serialize(),
+              'to satisfy',
+              {data: {list: [{herp: 'derp'}, {bing: 'bong'}]}}
+            );
+          });
+        });
+
+        describe('when passed an error', function() {
+          it('should serialize the error', function() {
+            const obj = {};
+            const err = new Error('monkeypants');
+            expect(
+              SerializableEvent.create('some-event', obj, err).serialize(),
+              'to satisfy',
+              {
+                eventName: 'some-event',
+                error: {
+                  message: 'monkeypants',
+                  stack: /^Error: monkeypants/,
+                  __type: 'Error'
+                },
+                data: obj
+              }
+            );
+          });
+
+          it('should retain own props', function() {
+            const obj = {};
+            const err = new Error('monkeypants');
+            err.code = 'MONKEY';
+            expect(
+              SerializableEvent.create('some-event', obj, err).serialize(),
+              'to satisfy',
+              {
+                eventName: 'some-event',
+                error: {
+                  code: 'MONKEY',
+                  message: 'monkeypants',
+                  stack: /^Error: monkeypants/,
+                  __type: 'Error'
+                },
+                data: obj
+              }
+            );
+          });
+
+          it('should not retain not-own props', function() {
+            const obj = {};
+            const err = new Error('monkeypants');
+            // eslint-disable-next-line no-proto
+            err.__proto__.code = 'MONKEY';
+            expect(
+              SerializableEvent.create('some-event', obj, err).serialize(),
+              'to satisfy',
+              {
+                eventName: 'some-event',
+                error: {
+                  message: 'monkeypants',
+                  stack: /^Error: monkeypants/,
+                  __type: 'Error'
+                },
+                data: obj
+              }
+            );
+          });
+        });
+
+        describe('when passed an object containing a top-level prop with an Error value', function() {
+          it('should serialize the Error', function() {
+            const obj = {
+              monkeyError: new Error('pantsmonkey')
+            };
+            const evt = SerializableEvent.create('some-event', obj);
+            expect(evt.serialize(), 'to satisfy', {
+              eventName: 'some-event',
+              data: {
+                monkeyError: {
+                  message: 'pantsmonkey',
+                  stack: /^Error: pantsmonkey/,
+                  __type: 'Error'
+                }
+              }
+            });
+          });
+        });
+        describe('when passed an object containing a nested prop with an Error value', function() {
+          it('should serialize the Error', function() {
+            const obj = {
+              nestedObj: {
+                monkeyError: new Error('pantsmonkey')
+              }
+            };
+            const evt = SerializableEvent.create('some-event', obj);
+            expect(evt.serialize(), 'to satisfy', {
+              eventName: 'some-event',
+              data: {
+                nestedObj: {
+                  monkeyError: {
+                    message: 'pantsmonkey',
+                    stack: /^Error: pantsmonkey/,
+                    __type: 'Error'
+                  }
+                }
+              }
+            });
+          });
+        });
+      });
+    });
+
+    describe('static method', function() {
+      describe('deserialize', function() {
+        describe('when passed a falsy parameter', function() {
+          it('should throw "invalid arg type" error', function() {
+            expect(SerializableEvent.deserialize, 'to throw', {
+              code: 'ERR_MOCHA_INVALID_ARG_TYPE'
+            });
+          });
+        });
+
+        it('should return a new object w/ null prototype', function() {
+          const obj = {bob: 'bob'};
+          expect(SerializableEvent.deserialize(obj), 'to satisfy', obj)
+            .and('not to equal', obj)
+            .and('not to have property', 'constructor');
+        });
+
+        describe('when passed value contains `data` prop', function() {
+          it('should ignore __proto__', function() {
+            const obj = {
+              data: Object.create(null)
+            };
+            // eslint-disable-next-line no-proto
+            obj.data.__proto__ = {peaches: 'prunes'};
+
+            const expected = Object.assign(Object.create(null), {
+              data: Object.create(null)
+            });
+            expect(SerializableEvent.deserialize(obj), 'to equal', expected);
+          });
+
+          describe('when `data` prop contains a nested serialized Error prop', function() {
+            it('should create an Error instance from the nested serialized Error prop', function() {
+              const message = 'problems!';
+              const stack = 'problem instructions';
+              const code = 'EIEIO';
+              const expected = Object.assign(Object.create(null), {
+                data: {
+                  whoops: Object.assign(new Error(message), {
+                    stack,
+                    code
+                  })
+                }
+              });
+
+              expect(
+                SerializableEvent.deserialize({
+                  data: {
+                    whoops: {
+                      message,
+                      stack,
+                      code,
+                      __type: 'Error'
+                    }
+                  }
+                }),
+                'to equal',
+                expected
+              );
+            });
+          });
+        });
+
+        describe('when passed value contains an `error` prop', function() {
+          it('should create an Error instance from the prop', function() {
+            const message = 'problems!';
+            const stack = 'problem instructions';
+            const code = 'EIEIO';
+            const expected = Object.assign(Object.create(null), {
+              error: Object.assign(new Error(message), {
+                stack,
+                code
+              })
+            });
+
+            expect(
+              SerializableEvent.deserialize({
+                error: {
+                  message,
+                  stack,
+                  code,
+                  __type: 'Error'
+                }
+              }),
+              'to equal',
+              expected
+            );
+          });
+        });
+
+        describe('when passed value data contains a prop beginning with "$$"', function() {
+          let result;
+
+          beforeEach(function() {
+            result = SerializableEvent.deserialize({data: {$$foo: 'bar'}});
+          });
+          it('should create a new prop having a function value', function() {
+            expect(result, 'to satisfy', {
+              data: {
+                foo: expect.it('to be a function')
+              }
+            });
+          });
+
+          it('should create a new prop returning the original value', function() {
+            expect(result.data.foo(), 'to equal', 'bar');
+          });
+
+          it('should remove the prop with the "$$" prefix', function() {
+            expect(result, 'not to have property', '$$foo');
+          });
+        });
+
+        describe('when the value data contains a prop with an array value', function() {
+          beforeEach(function() {
+            sandbox.spy(SerializableEvent, '_deserializeObject');
+          });
+
+          it('should deserialize each prop', function() {
+            const obj = {data: {foo: [{bar: 'baz'}]}};
+            SerializableEvent.deserialize(obj);
+            expect(
+              SerializableEvent._deserializeObject,
+              'to have a call satisfying',
+              {
+                args: [obj.data.foo, 0]
+              }
+            );
+          });
+        });
+      });
+
+      describe('create', function() {
+        it('should instantiate a SerializableEvent', function() {
+          expect(
+            SerializableEvent.create('some-event'),
+            'to be a',
+            SerializableEvent
+          );
+        });
+      });
+    });
+  });
+
+  describe('SerializableWorkerResult', function() {
+    describe('static method', function() {
+      describe('create', function() {
+        it('should return a new SerializableWorkerResult instance', function() {
+          expect(
+            SerializableWorkerResult.create(),
+            'to be a',
+            SerializableWorkerResult
+          );
+        });
+      });
+
+      describe('isSerializedWorkerResult', function() {
+        describe('when passed an instance', function() {
+          it('should return `true`', function() {
+            expect(
+              SerializableWorkerResult.isSerializedWorkerResult(
+                new SerializableWorkerResult()
+              ),
+              'to be true'
+            );
+          });
+        });
+
+        describe('when passed an object with an appropriate `__type` prop', function() {
+          it('should return `true`', function() {
+            // this is the most likely use-case, as the object is transmitted over IPC
+            // and loses its prototype
+            const original = new SerializableWorkerResult();
+            const clone = Object.assign({}, original);
+            expect(
+              SerializableWorkerResult.isSerializedWorkerResult(clone),
+              'to be true'
+            );
+          });
+        });
+
+        describe('when passed an object without an appropriate `__type` prop', function() {
+          it('should return `false`', function() {
+            expect(
+              SerializableWorkerResult.isSerializedWorkerResult({
+                mister: 'mister'
+              }),
+              'to be false'
+            );
+          });
+        });
+      });
+
+      describe('deserialize', function() {
+        beforeEach(function() {
+          sandbox.stub(SerializableEvent, 'deserialize');
+        });
+
+        it('should call SerializableEvent#deserialize on each item in its `events` prop', function() {
+          const result = Object.assign(
+            {},
+            SerializableWorkerResult.create([
+              {eventName: 'foo'},
+              {eventName: 'bar'}
+            ])
+          );
+          SerializableWorkerResult.deserialize(result);
+          expect(SerializableEvent.deserialize, 'to have calls satisfying', [
+            {args: [{eventName: 'foo'}]},
+            {args: [{eventName: 'bar'}]}
+          ]);
+        });
+
+        it('should return the deserialized value', function() {
+          const result = Object.assign(
+            {},
+            SerializableWorkerResult.create([
+              {eventName: 'foo'},
+              {eventName: 'bar'}
+            ])
+          );
+          expect(
+            SerializableWorkerResult.deserialize(result),
+            'to equal',
+            result
+          );
+        });
+      });
+    });
+
+    describe('instance method', function() {
+      describe('serialize', function() {
+        it('should return a read-only value', function() {
+          expect(
+            Object.isFrozen(SerializableWorkerResult.create().serialize()),
+            'to be true'
+          );
+        });
+
+        it('should call `SerializableEvent#serialize` of each of its events', function() {
+          sandbox.spy(SerializableEvent.prototype, 'serialize');
+          const events = [
+            SerializableEvent.create('foo'),
+            SerializableEvent.create('bar')
+          ];
+          SerializableWorkerResult.create(events).serialize();
+          expect(
+            SerializableEvent.prototype.serialize,
+            'to have calls satisfying',
+            [{thisValue: events[0]}, {thisValue: events[1]}]
+          );
+        });
+      });
+    });
+    describe('constructor', function() {
+      // the following two tests should be combined into one, but not sure how to express
+      // as a single assertion
+
+      it('should add a readonly `__type` prop', function() {
+        expect(
+          new SerializableWorkerResult(),
+          'to have readonly property',
+          '__type'
+        );
+      });
+    });
+  });
+});
diff --git a/test/node-unit/worker.spec.js b/test/node-unit/worker.spec.js
new file mode 100644
index 0000000000..94c3ceef18
--- /dev/null
+++ b/test/node-unit/worker.spec.js
@@ -0,0 +1,189 @@
+'use strict';
+
+const {SerializableWorkerResult} = require('../../lib/serializer');
+const rewiremock = require('rewiremock/node');
+const {createSandbox} = require('sinon');
+
+const WORKER_PATH = require.resolve('../../lib/worker.js');
+
+describe('worker', function() {
+  let worker;
+  let workerpoolWorker;
+  let sandbox;
+
+  beforeEach(function() {
+    sandbox = createSandbox();
+    workerpoolWorker = sandbox.stub();
+    sandbox.spy(process, 'removeAllListeners');
+  });
+
+  describe('when run as main "thread"', function() {
+    it('should throw', function() {
+      expect(() => {
+        rewiremock.proxy(WORKER_PATH, {
+          workerpool: {
+            isMainThread: true,
+            worker: workerpoolWorker
+          }
+        });
+      }, 'to throw');
+    });
+  });
+
+  describe('when run as "worker thread"', function() {
+    class MockMocha {}
+    let serializer;
+    let runHelpers;
+
+    beforeEach(function() {
+      MockMocha.prototype.addFile = sandbox.stub().returnsThis();
+      MockMocha.prototype.loadFilesAsync = sandbox.stub();
+      MockMocha.prototype.run = sandbox.stub();
+      MockMocha.interfaces = {
+        bdd: sandbox.stub()
+      };
+
+      serializer = {
+        serialize: sandbox.stub()
+      };
+
+      runHelpers = {
+        handleRequires: sandbox.stub(),
+        validatePlugin: sandbox.stub(),
+        loadRootHooks: sandbox.stub().resolves()
+      };
+
+      worker = rewiremock.proxy(WORKER_PATH, {
+        workerpool: {
+          isMainThread: false,
+          worker: workerpoolWorker
+        },
+        '../../lib/mocha': MockMocha,
+        '../../lib/serializer': serializer,
+        '../../lib/cli/run-helpers': runHelpers
+      });
+    });
+
+    it('should register itself with workerpool', function() {
+      expect(workerpoolWorker, 'to have a call satisfying', [
+        {run: worker.run}
+      ]);
+    });
+
+    describe('function', function() {
+      describe('run', function() {
+        describe('when called without arguments', function() {
+          it('should reject', async function() {
+            return expect(worker.run, 'to be rejected with error satisfying', {
+              code: 'ERR_MOCHA_INVALID_ARG_TYPE'
+            });
+          });
+        });
+
+        describe('when called with empty "filepath" argument', function() {
+          it('should reject', async function() {
+            return expect(
+              () => worker.run(''),
+              'to be rejected with error satisfying',
+              {
+                code: 'ERR_MOCHA_INVALID_ARG_TYPE'
+              }
+            );
+          });
+        });
+
+        describe('when the file at "filepath" argument is unloadable', function() {
+          it('should reject', async function() {
+            MockMocha.prototype.loadFilesAsync.rejects();
+            return expect(
+              () => worker.run('some-non-existent-file.js'),
+              'to be rejected'
+            );
+          });
+        });
+
+        describe('when the file at "filepath" is loadable', function() {
+          let result;
+          beforeEach(function() {
+            result = SerializableWorkerResult.create();
+
+            MockMocha.prototype.loadFilesAsync.resolves();
+            MockMocha.prototype.run.yields(result);
+          });
+
+          it('should handle "--require"', async function() {
+            await worker.run('some-file.js', {require: 'foo'});
+            expect(runHelpers.handleRequires, 'to have a call satisfying', [
+              'foo'
+            ]).and('was called once');
+          });
+
+          it('should handle "--ui"', async function() {
+            const argv = {};
+            await worker.run('some-file.js', argv);
+
+            expect(runHelpers.validatePlugin, 'to have a call satisfying', [
+              argv,
+              'ui',
+              MockMocha.interfaces
+            ]).and('was called once');
+          });
+
+          it('should call Mocha#run', async function() {
+            await worker.run('some-file.js');
+            expect(MockMocha.prototype.run, 'was called once');
+          });
+
+          it('should remove all uncaughtException listeners', async function() {
+            await worker.run('some-file.js');
+            expect(process.removeAllListeners, 'to have a call satisfying', [
+              'uncaughtException'
+            ]);
+          });
+
+          describe('when serialization succeeds', function() {
+            beforeEach(function() {
+              serializer.serialize.returnsArg(0);
+            });
+
+            it('should resolve with a SerializedWorkerResult', async function() {
+              return expect(
+                worker.run('some-file.js'),
+                'to be fulfilled with',
+                result
+              );
+            });
+          });
+
+          describe('when serialization fails', function() {
+            beforeEach(function() {
+              serializer.serialize.throws();
+            });
+
+            it('should reject', async function() {
+              return expect(worker.run('some-file.js'), 'to be rejected');
+            });
+          });
+
+          describe('when run twice', function() {
+            it('should initialize only once', async function() {
+              await worker.run('some-file.js');
+              await worker.run('some-other-file.js');
+
+              expect(runHelpers, 'to satisfy', {
+                handleRequires: expect.it('was called once'),
+                validatePlugin: expect.it('was called once')
+              });
+            });
+          });
+        });
+      });
+    });
+  });
+
+  afterEach(function() {
+    sandbox.restore();
+    // this is needed due to `require.cache` getting dumped in watch mode
+    process.removeAllListeners('beforeExit');
+  });
+});
diff --git a/test/reporters/buffered.spec.js b/test/reporters/buffered.spec.js
new file mode 100644
index 0000000000..981b939fd8
--- /dev/null
+++ b/test/reporters/buffered.spec.js
@@ -0,0 +1,230 @@
+'use strict';
+
+// this reporter does not actually output anything to the terminal, so we
+// need to test it differently.
+
+const {
+  EVENT_SUITE_BEGIN,
+  EVENT_SUITE_END,
+  EVENT_TEST_FAIL,
+  EVENT_TEST_PASS,
+  EVENT_TEST_PENDING,
+  EVENT_TEST_BEGIN,
+  EVENT_TEST_END,
+  EVENT_TEST_RETRY,
+  EVENT_DELAY_BEGIN,
+  EVENT_DELAY_END,
+  EVENT_HOOK_BEGIN,
+  EVENT_HOOK_END,
+  EVENT_RUN_END
+} = require('../../lib/runner').constants;
+const {EventEmitter} = require('events');
+const {createSandbox} = require('sinon');
+const rewiremock = require('rewiremock/node');
+
+describe('Buffered', function() {
+  let sandbox;
+  let runner;
+  let Buffered;
+
+  beforeEach(function() {
+    sandbox = createSandbox();
+    runner = new EventEmitter();
+    Buffered = rewiremock.proxy(
+      require.resolve('../../lib/reporters/buffered.js'),
+      {
+        '../../lib/serializer': {
+          SerializableEvent: {
+            create: (eventName, runnable, err) => ({
+              eventName,
+              data: runnable,
+              error: err,
+              __type: 'MockSerializableEvent'
+            })
+          },
+          SerializableWorkerResult: {
+            create: (events, failures) => ({
+              events,
+              failures,
+              __type: 'MockSerializableWorkerResult'
+            })
+          }
+        },
+        '../../lib/reporters/base': class MockBase {}
+      }
+    );
+  });
+
+  afterEach(function() {
+    sandbox.restore();
+  });
+
+  describe('constructor', function() {
+    it('should listen for Runner events', function() {
+      // EventEmitter#once calls thru to EventEmitter#on, which
+      // befouls our assertion below.
+      sandbox.stub(runner, 'once');
+      sandbox.stub(runner, 'on');
+      // eslint-disable-next-line no-new
+      new Buffered(runner);
+      expect(runner.on, 'to have calls satisfying', [
+        // via Buffered
+        [EVENT_SUITE_BEGIN, expect.it('to be a function')],
+        [EVENT_SUITE_END, expect.it('to be a function')],
+        [EVENT_TEST_BEGIN, expect.it('to be a function')],
+        [EVENT_TEST_PENDING, expect.it('to be a function')],
+        [EVENT_TEST_FAIL, expect.it('to be a function')],
+        [EVENT_TEST_PASS, expect.it('to be a function')],
+        [EVENT_TEST_RETRY, expect.it('to be a function')],
+        [EVENT_TEST_END, expect.it('to be a function')],
+        [EVENT_HOOK_BEGIN, expect.it('to be a function')],
+        [EVENT_HOOK_END, expect.it('to be a function')]
+      ]);
+    });
+
+    it('should listen for Runner events expecting to occur once', function() {
+      sandbox.stub(runner, 'once');
+      // eslint-disable-next-line no-new
+      new Buffered(runner);
+      expect(runner.once, 'to have calls satisfying', [
+        [EVENT_DELAY_BEGIN, expect.it('to be a function')],
+        [EVENT_DELAY_END, expect.it('to be a function')],
+        [EVENT_RUN_END, expect.it('to be a function')]
+      ]);
+    });
+  });
+
+  describe('event', function() {
+    let reporter;
+
+    beforeEach(function() {
+      reporter = new Buffered(runner);
+    });
+
+    describe('on EVENT_RUN_END', function() {
+      it('should remove all listeners', function() {
+        runner.emit(EVENT_RUN_END);
+        expect(runner.listeners(), 'to be empty');
+      });
+    });
+
+    describe('on any other event listened for', function() {
+      it('should populate its `events` array with SerializableEvents', function() {
+        const suite = {
+          title: 'some suite'
+        };
+        const test = {
+          title: 'some test'
+        };
+        runner.emit(EVENT_SUITE_BEGIN, suite);
+        runner.emit(EVENT_TEST_BEGIN, test);
+        runner.emit(EVENT_TEST_PASS, test);
+        runner.emit(EVENT_TEST_END, test);
+        runner.emit(EVENT_SUITE_END, suite);
+        expect(reporter.events, 'to equal', [
+          {
+            eventName: EVENT_SUITE_BEGIN,
+            data: suite,
+            __type: 'MockSerializableEvent'
+          },
+          {
+            eventName: EVENT_TEST_BEGIN,
+            data: test,
+            __type: 'MockSerializableEvent'
+          },
+          {
+            eventName: EVENT_TEST_PASS,
+            data: test,
+            __type: 'MockSerializableEvent'
+          },
+          {
+            eventName: EVENT_TEST_END,
+            data: test,
+            __type: 'MockSerializableEvent'
+          },
+          {
+            eventName: EVENT_SUITE_END,
+            data: suite,
+            __type: 'MockSerializableEvent'
+          }
+        ]);
+      });
+    });
+  });
+
+  describe('instance method', function() {
+    let reporter;
+
+    beforeEach(function() {
+      reporter = new Buffered(runner);
+    });
+
+    describe('done', function() {
+      it('should execute its callback with a SerializableWorkerResult', function() {
+        const suite = {
+          title: 'some suite'
+        };
+        const test = {
+          title: 'some test'
+        };
+        runner.emit(EVENT_SUITE_BEGIN, suite);
+        runner.emit(EVENT_TEST_BEGIN, test);
+        runner.emit(EVENT_TEST_PASS, test);
+        runner.emit(EVENT_TEST_END, test);
+        runner.emit(EVENT_SUITE_END, suite);
+        const cb = sandbox.stub();
+        reporter.done(0, cb);
+        expect(cb, 'to have a call satisfying', [
+          {
+            events: [
+              {
+                eventName: EVENT_SUITE_BEGIN,
+                data: suite,
+                __type: 'MockSerializableEvent'
+              },
+              {
+                eventName: EVENT_TEST_BEGIN,
+                data: test,
+                __type: 'MockSerializableEvent'
+              },
+              {
+                eventName: EVENT_TEST_PASS,
+                data: test,
+                __type: 'MockSerializableEvent'
+              },
+              {
+                eventName: EVENT_TEST_END,
+                data: test,
+                __type: 'MockSerializableEvent'
+              },
+              {
+                eventName: EVENT_SUITE_END,
+                data: suite,
+                __type: 'MockSerializableEvent'
+              }
+            ],
+            failures: 0,
+            __type: 'MockSerializableWorkerResult'
+          }
+        ]);
+      });
+
+      it('should reset its `events` prop', function() {
+        const suite = {
+          title: 'some suite'
+        };
+        const test = {
+          title: 'some test'
+        };
+        runner.emit(EVENT_SUITE_BEGIN, suite);
+        runner.emit(EVENT_TEST_BEGIN, test);
+        runner.emit(EVENT_TEST_PASS, test);
+        runner.emit(EVENT_TEST_END, test);
+        runner.emit(EVENT_SUITE_END, suite);
+        const cb = sandbox.stub();
+        reporter.done(0, cb);
+        expect(reporter.events, 'to be empty');
+      });
+    });
+  });
+});
diff --git a/test/reporters/tap.spec.js b/test/reporters/tap.spec.js
index f3bfe8d473..01294e0430 100644
--- a/test/reporters/tap.spec.js
+++ b/test/reporters/tap.spec.js
@@ -40,29 +40,17 @@ describe('TAP reporter', function() {
     describe('event handlers', function() {
       describe("on 'start' event", function() {
         var expectedSuite = 'some suite';
-        var expectedTotal = 10;
-        var expectedString;
         var stdout = [];
 
         before(function() {
           var runner = createMockRunner('start', EVENT_RUN_BEGIN);
           runner.suite = expectedSuite;
-          runner.grepTotal = function(string) {
-            expectedString = string;
-            return expectedTotal;
-          };
           stdout = runReporter({}, runner, options);
         });
 
         it('should not write a TAP specification version', function() {
           expect(stdout, 'not to contain', 'TAP version');
         });
-
-        it('should write the number of tests that it plans to run', function() {
-          var expectedArray = ['1..' + expectedTotal + '\n'];
-          expect(stdout, 'to equal', expectedArray);
-          expect(expectedString, 'to be', expectedSuite);
-        });
       });
 
       describe("on 'pending' event", function() {
@@ -78,7 +66,6 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
@@ -102,7 +89,6 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
@@ -141,7 +127,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -171,7 +156,6 @@ describe('TAP reporter', function() {
               error
             );
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -209,7 +193,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -245,7 +228,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -271,11 +253,10 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
-        it('should write total tests, passes, and failures', function() {
+        it('should write total tests, passes, failures, & plan', function() {
           var numberOfPasses = 1;
           var numberOfFails = 1;
           var totalTests = numberOfPasses + numberOfFails;
@@ -284,7 +265,8 @@ describe('TAP reporter', function() {
             'not ok ' + numberOfFails + ' ' + expectedTitle + '\n',
             '# tests ' + totalTests + '\n',
             '# pass ' + numberOfPasses + '\n',
-            '# fail ' + numberOfFails + '\n'
+            '# fail ' + numberOfFails + '\n',
+            '1..' + totalTests + '\n'
           ];
           expect(stdout, 'to equal', expectedArray);
         });
@@ -302,17 +284,11 @@ describe('TAP reporter', function() {
     describe('event handlers', function() {
       describe("on 'start' event", function() {
         var expectedSuite = 'some suite';
-        var expectedTotal = 10;
-        var expectedString;
         var stdout;
 
         before(function() {
           var runner = createMockRunner('start', EVENT_RUN_BEGIN);
           runner.suite = expectedSuite;
-          runner.grepTotal = function(string) {
-            expectedString = string;
-            return expectedTotal;
-          };
           stdout = runReporter({}, runner, options);
         });
 
@@ -321,12 +297,6 @@ describe('TAP reporter', function() {
           var expectedFirstLine = 'TAP version ' + tapVersion + '\n';
           expect(stdout[0], 'to equal', expectedFirstLine);
         });
-
-        it('should write the number of tests that it plans to run', function() {
-          var expectedSecondLine = '1..' + expectedTotal + '\n';
-          expect(stdout[1], 'to equal', expectedSecondLine);
-          expect(expectedString, 'to be', expectedSuite);
-        });
       });
 
       describe("on 'pending' event", function() {
@@ -342,7 +312,6 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
@@ -366,7 +335,6 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
@@ -405,7 +373,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -438,7 +405,6 @@ describe('TAP reporter', function() {
               error
             );
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -479,7 +445,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -519,7 +484,6 @@ describe('TAP reporter', function() {
               }
             };
             runner.suite = '';
-            runner.grepTotal = noop;
             stdout = runReporter({}, runner, options);
           });
 
@@ -545,11 +509,10 @@ describe('TAP reporter', function() {
             test
           );
           runner.suite = '';
-          runner.grepTotal = noop;
           stdout = runReporter({}, runner, options);
         });
 
-        it('should write total tests, passes, and failures', function() {
+        it('should write total tests, passes, failures & plan', function() {
           var numberOfPasses = 1;
           var numberOfFails = 1;
           var totalTests = numberOfPasses + numberOfFails;
@@ -558,7 +521,8 @@ describe('TAP reporter', function() {
             'not ok ' + numberOfFails + ' ' + expectedTitle + '\n',
             '# tests ' + totalTests + '\n',
             '# pass ' + numberOfPasses + '\n',
-            '# fail ' + numberOfFails + '\n'
+            '# fail ' + numberOfFails + '\n',
+            '1..' + totalTests + '\n'
           ];
           expect(stdout, 'to equal', expectedArray);
         });