diff --git a/lib/groupBy.js b/lib/groupBy.js
new file mode 100644
index 000000000..31e2c0b75
--- /dev/null
+++ b/lib/groupBy.js
@@ -0,0 +1,40 @@
+import doLimit from './internal/doLimit';
+import groupByLimit from './groupByLimit';
+
+/**
+ * Returns a new object, where each value corresponds to an array of items, from
+ * `coll`, that returned the corresponding key. That is, the keys of the object
+ * correspond to the values passed to the `iteratee` callback.
+ *
+ * Note: Since this function applies the `iteratee` to each item in parallel,
+ * there is no guarantee that the `iteratee` functions will complete in order.
+ * However, the values for each key in the `result` will be in the same order as
+ * the original `coll`. For Objects, the values will roughly be in the order of
+ * the original Objects' keys (but this can vary across JavaScript engines).
+ *
+ * @name groupBy
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @category Collection
+ * @param {Array|Iterable|Object} coll - A collection to iterate over.
+ * @param {Function} iteratee - A function to apply to each item in `coll`.
+ * The iteratee is passed a `callback(err, key)` which must be called once it
+ * has completed with an error (which can be `null`) and the `key` to group the
+ * value under. Invoked with (value, callback).
+ * @param {Function} [callback] - A callback which is called when all `iteratee`
+ * functions have finished, or an error occurs. Result is an `Object` whoses
+ * properties are arrays of values which returned the corresponding key.
+ * @example
+ *
+ * async.groupBy(['userId1', 'userId2', 'userId3'], function(userId, callback) {
+ *     db.findById(userId, function(err, user) {
+ *         if (err) return callback(err);
+ *         return callback(null, user.age);
+ *     });
+ * }, function(err, result) {
+ *     // result is object containing the userIds grouped by age
+ *     // e.g. { 30: ['userId1', 'userId3'], 42: ['userId2']};
+ * });
+ */
+export default doLimit(groupByLimit, Infinity);
diff --git a/lib/groupByLimit.js b/lib/groupByLimit.js
new file mode 100644
index 000000000..9fc71a0c3
--- /dev/null
+++ b/lib/groupByLimit.js
@@ -0,0 +1,51 @@
+import noop from 'lodash/noop';
+import mapLimit from './mapLimit';
+
+/**
+ * The same as [`groupBy`]{@link module:Collections.groupBy} but runs a maximum of `limit` async operations at a time.
+ *
+ * @name groupByLimit
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.groupBy]{@link module:Collections.groupBy}
+ * @category Collection
+ * @param {Array|Iterable|Object} coll - A collection to iterate over.
+ * @param {number} limit - The maximum number of async operations at a time.
+ * @param {Function} iteratee - A function to apply to each item in `coll`.
+ * The iteratee is passed a `callback(err, key)` which must be called once it
+ * has completed with an error (which can be `null`) and the `key` to group the
+ * value under. Invoked with (value, callback).
+ * @param {Function} [callback] - A callback which is called when all `iteratee`
+ * functions have finished, or an error occurs. Result is an `Object` whoses
+ * properties are arrays of values which returned the corresponding key.
+ */
+export default function(coll, limit, iteratee, callback) {
+    callback = callback || noop;
+
+    mapLimit(coll, limit, function(val, callback) {
+        iteratee(val, function(err, key) {
+            if (err) return callback(err);
+            return callback(null, {key: key, val: val});
+        });
+    }, function(err, mapResults) {
+        var result = {};
+        // from MDN, handle object having an `hasOwnProperty` prop
+        var hasOwnProperty = Object.prototype.hasOwnProperty;
+
+        for (var i = 0; i < mapResults.length; i++) {
+            if (mapResults[i]) {
+                var key = mapResults[i].key;
+                var val = mapResults[i].val;
+
+                if (hasOwnProperty.call(result, key)) {
+                    result[key].push(val);
+                } else {
+                    result[key] = [val];
+                }
+            }
+        }
+
+        return callback(err, result);
+    });
+};
diff --git a/lib/groupBySeries.js b/lib/groupBySeries.js
new file mode 100644
index 000000000..5df9a3cbe
--- /dev/null
+++ b/lib/groupBySeries.js
@@ -0,0 +1,23 @@
+import doLimit from './internal/doLimit';
+import groupByLimit from './groupByLimit';
+
+/**
+ * The same as [`groupBy`]{@link module:Collections.groupBy} but runs only a single async operation at a time.
+ *
+ * @name groupBySeries
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.groupBy]{@link module:Collections.groupBy}
+ * @category Collection
+ * @param {Array|Iterable|Object} coll - A collection to iterate over.
+ * @param {number} limit - The maximum number of async operations at a time.
+ * @param {Function} iteratee - A function to apply to each item in `coll`.
+ * The iteratee is passed a `callback(err, key)` which must be called once it
+ * has completed with an error (which can be `null`) and the `key` to group the
+ * value under. Invoked with (value, callback).
+ * @param {Function} [callback] - A callback which is called when all `iteratee`
+ * functions have finished, or an error occurs. Result is an `Object` whoses
+ * properties are arrays of values which returned the corresponding key.
+ */
+export default doLimit(groupByLimit, 1);
diff --git a/lib/index.js b/lib/index.js
index 8a2c7cad9..0bb7efe19 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -54,6 +54,9 @@ import filter from './filter';
 import filterLimit from './filterLimit';
 import filterSeries from './filterSeries';
 import forever from './forever';
+import groupBy from './groupBy';
+import groupByLimit from './groupByLimit';
+import groupBySeries from './groupBySeries';
 import log from './log';
 import map from './map';
 import mapLimit from './mapLimit';
@@ -128,6 +131,9 @@ export default {
     filterLimit: filterLimit,
     filterSeries: filterSeries,
     forever: forever,
+    groupBy: groupBy,
+    groupByLimit: groupByLimit,
+    groupBySeries: groupBySeries,
     log: log,
     map: map,
     mapLimit: mapLimit,
@@ -220,6 +226,9 @@ export {
     filterLimit as filterLimit,
     filterSeries as filterSeries,
     forever as forever,
+    groupBy as groupBy,
+    groupByLimit as groupByLimit,
+    groupBySeries as groupBySeries,
     log as log,
     map as map,
     mapLimit as mapLimit,
diff --git a/lib/map.js b/lib/map.js
index 4c0b22685..76d60582b 100644
--- a/lib/map.js
+++ b/lib/map.js
@@ -16,7 +16,7 @@ import map from './internal/map';
  *
  * If `map` is passed an Object, the results will be an Array.  The results
  * will roughly be in the order of the original Objects' keys (but this can
- * vary across JavaScript engines)
+ * vary across JavaScript engines).
  *
  * @name map
  * @static
diff --git a/mocha_test/groupBy.js b/mocha_test/groupBy.js
new file mode 100644
index 000000000..afb612b0c
--- /dev/null
+++ b/mocha_test/groupBy.js
@@ -0,0 +1,354 @@
+var async = require('../lib');
+var expect = require('chai').expect;
+var assert = require('assert');
+
+describe('groupBy', function() {
+    this.timeout(250);
+
+    function groupByIteratee(callOrder, val, next) {
+        setTimeout(function() {
+            callOrder.push(val);
+            next(null, val+1);
+        }, val * 25);
+    }
+
+    context('groupBy', function() {
+        it('basics', function(done) {
+            var callOrder = [];
+            async.groupBy([1, 3, 2], groupByIteratee.bind(this, callOrder), function(err, result) {
+                expect(err).to.eql(null);
+                expect(callOrder).to.eql([1, 2, 3]);
+                expect(result).to.eql({2: [1], 3: [2], 4: [3]});
+                done();
+            });
+        });
+
+        it('error', function(done) {
+            async.groupBy([1, 3, 2], function(val, next) {
+                if (val === 3) {
+                    return next(new Error('fail'));
+                }
+                next(null, val+1);
+            }, function(err, result) {
+                expect(err).to.not.eql(null);
+                expect(result).to.eql({2: [1]});
+                done();
+            });
+        });
+
+        it('original untouched', function(done) {
+            var obj = {a: 'b', b: 'c', c: 'd'};
+            async.groupBy(obj, function(val, next) {
+                next(null, val);
+            }, function(err, result) {
+                expect(obj).to.eql({a: 'b', b: 'c', c: 'd'});
+                expect(result).to.eql({b: ['b'], c: ['c'], d: ['d']});
+                done();
+            });
+        });
+
+        it('handles multiple matches', function(done) {
+            var callOrder = [];
+            async.groupBy([1, 3, 2, 2], groupByIteratee.bind(this, callOrder), function(err, result) {
+                expect(err).to.eql(null);
+                expect(callOrder).to.eql([1, 2, 2, 3]);
+                expect(result).to.eql({2: [1], 3: [2, 2], 4: [3]});
+                done();
+            });
+        });
+
+        it('handles objects', function(done) {
+            var obj = {a: 'b', b: 'c', c: 'd'};
+            var concurrency = {b: 3, c: 2, d: 1};
+            var running = 0;
+            async.groupBy(obj, function(val, next) {
+                running++;
+                async.setImmediate(function() {
+                    expect(running).to.equal(concurrency[val]);
+                    running--;
+                    next(null, val);
+                });
+            }, function(err, result) {
+                expect(running).to.equal(0);
+                expect(err).to.eql(null);
+                expect(result).to.eql({b: ['b'], c: ['c'], d: ['d']});
+                done();
+            });
+        });
+
+        it('handles undefined', function(done) {
+            async.groupBy(undefined, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('handles empty object', function(done) {
+            async.groupBy({}, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('main callback optional' , function(done) {
+            var arr = [1, 2, 3];
+            var runs = [];
+            async.groupBy(arr, function(val, next) {
+                runs.push(val);
+                var _done = (runs.length === arr.length);
+                async.setImmediate(function() {
+                    next(null);
+                    if (_done) {
+                        expect(runs).to.eql(arr);
+                        done();
+                    }
+                });
+            });
+        });
+
+        it('iteratee callback is only called once', function(done) {
+            async.groupBy([1, 2], function(item, callback) {
+                try {
+                    callback(item);
+                } catch (exception) {
+                    expect(function() {
+                        callback(exception);
+                    }).to.throw(/already called/);
+                    done();
+                }
+            }, function() {
+                throw new Error();
+            });
+        });
+
+        it('handles Map', function(done) {
+            if (typeof Map !== 'function') {
+                return done();
+            }
+
+            var map = new Map([
+                ['a', 'a'],
+                ['b', 'b'],
+                ['c', 'a']
+            ]);
+
+            async.groupBy(map, function(val, next) {
+                next(null, val[1]+1);
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({
+                    a1: [ ['a', 'a'], ['c', 'a']],
+                    b1: [ ['b', 'b'] ]
+                });
+                done();
+            });
+        });
+
+        it('handles sparse results', function(done) {
+            var arr = [1, 2, 3];
+            async.groupBy(arr, function(val, next) {
+                if (val === 1) {
+                    return next(null, val+1);
+                } else if (val === 2) {
+                    async.setImmediate(function() {
+                        return next(null, val+1);
+                    });
+                } else {
+                    return next('error');
+                }
+            }, function(err, result) {
+                expect(err).to.not.eql(null);
+                expect(result).to.eql({2: [1]});
+                async.setImmediate(done);
+            });
+        });
+    });
+
+    context('groupByLimit', function() {
+        var obj = {a: 'b', b: 'c', c: 'd'};
+        it('basics', function(done) {
+            var running = 0;
+            var concurrency = {'b': 2, 'c': 2, 'd': 1};
+            async.groupByLimit(obj, 2, function(val, next) {
+                running++;
+                async.setImmediate(function() {
+                    expect(running).to.equal(concurrency[val]);
+                    running--;
+                    next(null, val);
+                });
+            }, function(err, result) {
+                expect(running).to.equal(0);
+                expect(err).to.eql(null);
+                expect(result).to.eql({'b': ['b'], 'c': ['c'], 'd': ['d']})
+                done();
+            });
+        });
+
+        it('error', function(done) {
+            async.groupByLimit(obj, 1, function(val, next) {
+                if (val === 'c') {
+                    return next(new Error('fail'));
+                }
+                next(null, val);
+            }, function(err, result) {
+                expect(err).to.not.eql(null);
+                expect(result).to.eql({b: ['b']});
+                done();
+            });
+        });
+
+        it('handles empty object', function(done) {
+            async.groupByLimit({}, 2, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('handles undefined', function(done) {
+            async.groupByLimit(undefined, 2, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('limit exceeds size', function(done) {
+            var callOrder = [];
+            async.groupByLimit([3, 2, 2, 1], 10, groupByIteratee.bind(this, callOrder), function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({2: [1], 3: [2, 2], 4: [3]});
+                expect(callOrder).to.eql([1, 2, 2, 3]);
+                done();
+            });
+        });
+
+        it('limit equal size', function(done) {
+            var callOrder = [];
+            async.groupByLimit([3, 2, 2, 1], 4, groupByIteratee.bind(this, callOrder), function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({2: [1], 3: [2, 2], 4: [3]});
+                expect(callOrder).to.eql([1, 2, 2, 3]);
+                done();
+            });
+        });
+
+        it('zero limit', function(done) {
+            async.groupByLimit([3, 2, 2, 1], 0, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('does not continue replenishing after error', function(done) {
+            var started = 0;
+            var arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+            var delay = 10;
+            var limit = 3;
+            var maxTime = 10 * arr.length;
+
+            async.groupByLimit(arr, limit, function(val, next) {
+                started++;
+                if (started === 3) {
+                    return next(new Error('fail'));
+                }
+
+                setTimeout(function() {
+                    next();
+                }, delay);
+            }, function(err, result) {
+                expect(err).to.not.eql(null);
+                expect(result).to.eql({});
+            });
+
+            setTimeout(function() {
+                expect(started).to.equal(3);
+                done();
+            }, maxTime);
+        });
+    });
+
+    context('groupBySeries', function() {
+        var obj = {a: 'b', b: 'c', c: 'd'};
+        it('basics', function(done) {
+            var running = 0;
+            var concurrency = {'b': 1, 'c': 1, 'd': 1};
+            async.groupBySeries(obj, function(val, next) {
+                running++;
+                async.setImmediate(function() {
+                    expect(running).to.equal(concurrency[val]);
+                    running--;
+                    next(null, val);
+                });
+            }, function(err, result) {
+                expect(running).to.equal(0);
+                expect(err).to.eql(null);
+                expect(result).to.eql({'b': ['b'], 'c': ['c'], 'd': ['d']});
+                done();
+            });
+        });
+
+        it('error', function(done) {
+            async.groupBySeries(obj, function(val, next) {
+                if (val === 'c') {
+                    return next(new Error('fail'));
+                }
+                next(null, val);
+            }, function(err, result) {
+                expect(err).to.not.eql(null);
+                expect(result).to.eql({b: ['b']});
+                done();
+            });
+        });
+
+        it('handles arrays', function(done) {
+            async.groupBySeries(['a', 'a', 'b'], function(val, next) {
+                next(null, val);
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({'a': ['a', 'a'], 'b': ['b']});
+                done();
+            });
+        });
+
+        it('handles empty object', function(done) {
+            async.groupByLimit({}, 2, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+
+        it('handles undefined', function(done) {
+            async.groupBySeries(undefined, function(val, next) {
+                assert(false, 'iteratee should not be called');
+                next();
+            }, function(err, result) {
+                expect(err).to.eql(null);
+                expect(result).to.eql({});
+                done();
+            });
+        });
+    });
+});