diff --git a/bin/nyc.js b/bin/nyc.js
index c285c5abe..99c667e0d 100755
--- a/bin/nyc.js
+++ b/bin/nyc.js
@@ -65,6 +65,10 @@ if ([
   ), function (done) {
     var mainChildExitCode = process.exitCode
 
+    if (argv.showProcessTree || argv.buildProcessTree) {
+      nyc.writeProcessIndex()
+    }
+
     if (argv.checkCoverage) {
       checkCoverage(argv)
       process.exitCode = process.exitCode || mainChildExitCode
diff --git a/bin/wrap.js b/bin/wrap.js
index b4c916156..c15862c47 100644
--- a/bin/wrap.js
+++ b/bin/wrap.js
@@ -12,6 +12,10 @@ config._processInfo = {
   ppid: parentPid,
   root: process.env.NYC_ROOT_ID
 }
+if (process.env.NYC_PROCESSINFO_EXTERNAL_ID) {
+  config._processInfo.externalId = process.env.NYC_PROCESSINFO_EXTERNAL_ID
+  delete process.env.NYC_PROCESSINFO_EXTERNAL_ID
+}
 
 ;(new NYC(config)).wrap()
 
diff --git a/index.js b/index.js
index 00e6cc81f..aaf8c21f9 100755
--- a/index.js
+++ b/index.js
@@ -413,6 +413,105 @@ NYC.prototype.report = function () {
   }
 }
 
+// XXX(@isaacs) Index generation should move to istanbul-lib-processinfo
+NYC.prototype.writeProcessIndex = function () {
+  const dir = this.processInfoDirectory()
+  const pidToUid = new Map()
+  const infoByUid = new Map()
+  const eidToUid = new Map()
+  const infos = fs.readdirSync(dir).filter(f => f !== 'index.json').map(f => {
+    try {
+      const info = JSON.parse(fs.readFileSync(path.resolve(dir, f), 'utf-8'))
+      // on thiis first read pass, also map the pids to uuids
+      info.uuid = path.basename(f, '.json')
+      pidToUid.set(info.uuid, info.pid)
+      pidToUid.set(info.pid, info.uuid)
+      infoByUid.set(info.uuid, info)
+      if (info.externalId) {
+        eidToUid.set(info.externalId, info.uuid)
+      }
+      return info
+    } catch (er) {
+      return null
+    }
+  }).filter(Boolean)
+
+  // create all the parent-child links and write back the updated info
+  const needsUpdate = new Set()
+  infos.forEach(info => {
+    if (info.ppid && info.ppid !== '0' && !info.parent) {
+      info.parent = pidToUid.get(info.ppid)
+      needsUpdate.add(info)
+    }
+    if (info.parent) {
+      const parentInfo = infoByUid.get(info.parent)
+      if (parentInfo.children.indexOf(info.uuid) === -1) {
+        parentInfo.children.push(info.uuid)
+        needsUpdate.add(parentInfo)
+      }
+    }
+  })
+
+  // figure out which files were touched by each process.
+  const files = infos.reduce((files, info) => {
+    if (!info.files) {
+      try {
+        info.files = Object.keys(JSON.parse(fs.readFileSync(
+          path.resolve(this.tempDirectory(), info.coverageFilename),
+          'utf-8'
+        )))
+      } catch (er) {
+        return files
+      }
+      needsUpdate.add(info)
+    }
+    info.files.forEach(f => {
+      files[f] = files[f] || []
+      files[f].push(info.uuid)
+    })
+    return files
+  }, {})
+
+  // build the actual index!
+  const index = infos.reduce((index, info) => {
+    index.processes[info.uuid] = {}
+    if (info.parent) {
+      index.processes[info.uuid].parent = info.parent
+    }
+    if (info.externalId) {
+      index.processes[info.uuid].externalId = info.externalId
+      index.externalIds[info.externalId] = {
+        root: info.uuid,
+        children: info.children
+      }
+    }
+    if (info.children && info.children.length) {
+      index.processes[info.uuid].children = Array.from(info.children)
+    }
+    return index
+  }, { processes: {}, files: files, externalIds: {} })
+
+  // flatten the descendant sets of all the externalId procs
+  Object.keys(index.externalIds).forEach(eid => {
+    const { children } = index.externalIds[eid]
+    // push the next generation onto the list so we accumulate them all
+    for (let i = 0; i < children.length; i++) {
+      const nextGen = index.processes[children[i]].children
+      if (nextGen && nextGen.length) {
+        children.push(...nextGen.filter(uuid => children.indexOf(uuid) === -1))
+      }
+    }
+  })
+
+  needsUpdate.forEach(info => {
+    fs.writeFileSync(
+      path.resolve(dir, info.uuid + '.json'), JSON.stringify(info)
+    )
+  })
+
+  fs.writeFileSync(path.resolve(dir, 'index.json'), JSON.stringify(index))
+}
+
 NYC.prototype.showProcessTree = function () {
   var processTree = ProcessInfo.buildProcessTree(this._loadProcessInfos())
 
@@ -452,7 +551,7 @@ NYC.prototype._loadProcessInfos = function () {
   var _this = this
   var files = fs.readdirSync(this.processInfoDirectory())
 
-  return files.map(function (f) {
+  return files.filter(f => f !== 'index.json').map(function (f) {
     try {
       return new ProcessInfo(JSON.parse(fs.readFileSync(
         path.resolve(_this.processInfoDirectory(), f),
diff --git a/lib/process.js b/lib/process.js
index c77ca503f..fb62ab6a9 100644
--- a/lib/process.js
+++ b/lib/process.js
@@ -13,6 +13,7 @@ function ProcessInfo (defaults) {
   this.root = null
   this.coverageFilename = null
   this.nodes = [] // list of children, filled by buildProcessTree()
+  this.children = [] // just uuids, not full nodes
 
   this._coverageMap = null