diff --git a/__tests__/List.ts b/__tests__/List.ts
index f1a8dd7013..f91f7a859f 100644
--- a/__tests__/List.ts
+++ b/__tests__/List.ts
@@ -642,12 +642,36 @@ describe('List', () => {
   });
 
   it('concat works like Array.prototype.concat', () => {
-    const v1 = List.of(1, 2, 3);
+    const v1 = List([1, 2, 3]);
     const v2 = v1.concat(4, List([ 5, 6 ]), [7, 8], Seq([ 9, 10 ]), Set.of(11, 12), null as any);
     expect(v1.toArray()).toEqual([1, 2, 3]);
     expect(v2.toArray()).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, null]);
   });
 
+  it('concat returns self when no changes', () => {
+    const v1 = List([1, 2, 3]);
+    expect(v1.concat([])).toBe(v1);
+  });
+
+  it('concat returns arg when concat to empty', () => {
+    const v1 = List([1, 2, 3]);
+    expect(List().concat(v1)).toBe(v1);
+  });
+
+  it('concats a single value', () => {
+    const v1 = List([1, 2, 3]);
+    expect(v1.concat(4)).toEqual(List([1, 2, 3, 4]));
+  });
+
+  it('concat returns List-coerced arg when concat to empty', () => {
+    expect(List().concat([1, 2, 3])).toEqual(List([1, 2, 3]));
+  });
+
+  it('concat does not spread in string characters', () => {
+    const v1 = List([1, 2, 3]);
+    expect(v1.concat('abcdef')).toEqual(List([1, 2, 3, 'abcdef']));
+  });
+
   it('allows chained mutations', () => {
     const v1 = List();
     const v2 = v1.push(1);
diff --git a/src/List.js b/src/List.js
index 4825962903..fda05e20e6 100644
--- a/src/List.js
+++ b/src/List.js
@@ -21,7 +21,7 @@ import {
 } from './TrieUtils';
 import { IndexedCollection } from './Collection';
 import { MapPrototype } from './Map';
-import { Iterator, iteratorValue, iteratorDone } from './Iterator';
+import { hasIterator, Iterator, iteratorValue, iteratorDone } from './Iterator';
 
 import assertNotInfinite from './utils/assertNotInfinite';
 
@@ -135,8 +135,28 @@ export class List extends IndexedCollection {
 
   // @pragma Composition
 
-  merge(/*...collections*/) {
-    return this.concat.apply(this, arguments);
+  concat(/*...collections*/) {
+    const seqs = [];
+    for (let i = 0; i < arguments.length; i++) {
+      const argument = arguments[i];
+      const seq = IndexedCollection(
+        typeof argument !== 'string' && hasIterator(argument)
+          ? argument
+          : [argument]
+      );
+      if (seq.size !== 0) {
+        seqs.push(seq);
+      }
+    }
+    if (seqs.length === 0) {
+      return this;
+    }
+    if (this.size === 0 && !this.__ownerID && seqs.length === 1) {
+      return this.constructor(seqs[0]);
+    }
+    return this.withMutations(list => {
+      seqs.forEach(seq => seq.forEach(value => list.push(value)));
+    });
   }
 
   setSize(size) {
@@ -215,6 +235,7 @@ const IS_LIST_SENTINEL = '@@__IMMUTABLE_LIST__@@';
 export const ListPrototype = List.prototype;
 ListPrototype[IS_LIST_SENTINEL] = true;
 ListPrototype[DELETE] = ListPrototype.remove;
+ListPrototype.merge = ListPrototype.concat;
 ListPrototype.setIn = MapPrototype.setIn;
 ListPrototype.deleteIn = ListPrototype.removeIn = MapPrototype.removeIn;
 ListPrototype.update = MapPrototype.update;
diff --git a/src/Map.js b/src/Map.js
index e354243da7..09e884ea7c 100644
--- a/src/Map.js
+++ b/src/Map.js
@@ -250,6 +250,7 @@ MapPrototype[IS_MAP_SENTINEL] = true;
 MapPrototype[DELETE] = MapPrototype.remove;
 MapPrototype.removeIn = MapPrototype.deleteIn;
 MapPrototype.removeAll = MapPrototype.deleteAll;
+MapPrototype.concat = MapPrototype.merge;
 MapPrototype['@@transducer/init'] = MapPrototype.asMutable;
 MapPrototype['@@transducer/step'] = function(result, arr) {
   return result.set(arr[0], arr[1]);
diff --git a/src/Set.js b/src/Set.js
index f1bfd74f6c..d1658ede45 100644
--- a/src/Set.js
+++ b/src/Set.js
@@ -178,7 +178,7 @@ const IS_SET_SENTINEL = '@@__IMMUTABLE_SET__@@';
 const SetPrototype = Set.prototype;
 SetPrototype[IS_SET_SENTINEL] = true;
 SetPrototype[DELETE] = SetPrototype.remove;
-SetPrototype.merge = SetPrototype.union;
+SetPrototype.merge = SetPrototype.concat = SetPrototype.union;
 SetPrototype.withMutations = MapPrototype.withMutations;
 SetPrototype.asMutable = MapPrototype.asMutable;
 SetPrototype.asImmutable = MapPrototype.asImmutable;
diff --git a/type-definitions/Immutable.d.ts b/type-definitions/Immutable.d.ts
index bf3de10a7d..e9900c67b4 100644
--- a/type-definitions/Immutable.d.ts
+++ b/type-definitions/Immutable.d.ts
@@ -787,7 +787,7 @@ declare module Immutable {
     /**
      * Returns a new List with other values or collections concatenated to this one.
      *
-     * Note: `concat` *cannot* be safely used in `withMutations`.
+     * Note: `concat` can be used in `withMutations`.
      *
      * @alias merge
      */
@@ -1223,8 +1223,13 @@ declare module Immutable {
      * ```
      *
      * Note: `merge` can be used in `withMutations`.
+     *
+     * @alias concat
      */
-    merge(...collections: Array<Iterable<[K, V]> | {[key: string]: V}>): this;
+    merge<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): Map<K | KC, V | VC>;
+    merge<C>(...collections: Array<{[key: string]: C}>): Map<K | string, V | C>;
+    concat<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): Map<K | KC, V | VC>;
+    concat<C>(...collections: Array<{[key: string]: C}>): Map<K | string, V | C>;
 
     /**
      * Like `merge()`, `mergeWith()` returns a new Map resulting from merging
@@ -1512,12 +1517,6 @@ declare module Immutable {
 
     // Sequence algorithms
 
-    /**
-     * Returns a new Map with other collections concatenated to this one.
-     */
-    concat<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): Map<K | KC, V | VC>;
-    concat<C>(...collections: Array<{[key: string]: C}>): Map<K | string, V | C>;
-
     /**
      * Returns a new Map with values passed through a
      * `mapper` function.
@@ -1628,14 +1627,34 @@ declare module Immutable {
      */
     readonly size: number;
 
-    // Sequence algorithms
-
     /**
-     * Returns a new OrderedMap with other collections concatenated to this one.
+     * Returns a new OrderedMap resulting from merging the provided Collections
+     * (or JS objects) into this OrderedMap. In other words, this takes each
+     * entry of each collection and sets it on this OrderedMap.
+     *
+     * Note: Values provided to `merge` are shallowly converted before being
+     * merged. No nested values are altered.
+     *
+     * <!-- runkit:activate -->
+     * ```js
+     * const { OrderedMap } = require('immutable@4.0.0-rc.7')
+     * const one = OrderedMap({ a: 10, b: 20, c: 30 })
+     * const two = OrderedMap({ b: 40, a: 50, d: 60 })
+     * one.merge(two) // OrderedMap { "a": 50, "b": 40, "c": 30, "d": 60 }
+     * two.merge(one) // OrderedMap { "b": 20, "a": 10, "d": 60, "c": 30 }
+     * ```
+     *
+     * Note: `merge` can be used in `withMutations`.
+     *
+     * @alias concat
      */
+    merge<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): OrderedMap<K | KC, V | VC>;
+    merge<C>(...collections: Array<{[key: string]: C}>): OrderedMap<K | string, V | C>;
     concat<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): OrderedMap<K | KC, V | VC>;
     concat<C>(...collections: Array<{[key: string]: C}>): OrderedMap<K | string, V | C>;
 
+    // Sequence algorithms
+
     /**
      * Returns a new OrderedMap with values passed through a
      * `mapper` function.
@@ -1811,9 +1830,11 @@ declare module Immutable {
      *
      * Note: `union` can be used in `withMutations`.
      * @alias merge
+     * @alias concat
      */
-    union(...collections: Array<Iterable<T>>): this;
-    merge(...collections: Array<Iterable<T>>): this;
+    union<C>(...collections: Array<Iterable<C>>): Set<T | C>;
+    merge<C>(...collections: Array<Iterable<C>>): Set<T | C>;
+    concat<C>(...collections: Array<Iterable<C>>): Set<T | C>;
 
     /**
      * Returns a Set which has removed any values not also contained
@@ -1821,14 +1842,14 @@ declare module Immutable {
      *
      * Note: `intersect` can be used in `withMutations`.
      */
-    intersect(...collections: Array<Collection<any, T> | Array<T>>): this;
+    intersect(...collections: Array<Iterable<T>>): this;
 
     /**
      * Returns a Set excluding any values contained within `collections`.
      *
      * Note: `subtract` can be used in `withMutations`.
      */
-    subtract(...collections: Array<Collection<any, T> | Array<T>>): this;
+    subtract(...collections: Array<Iterable<T>>): this;
 
 
     // Transient changes
@@ -1863,11 +1884,6 @@ declare module Immutable {
 
     // Sequence algorithms
 
-    /**
-     * Returns a new Set with other collections concatenated to this one.
-     */
-    concat<C>(...valuesOrCollections: Array<Iterable<C> | C>): Set<T | C>;
-
     /**
      * Returns a new Set with values passed through a
      * `mapper` function.
@@ -1956,12 +1972,19 @@ declare module Immutable {
      */
     readonly size: number;
 
-    // Sequence algorithms
-
     /**
-     * Returns a new OrderedSet with other collections concatenated to this one.
+     * Returns an OrderedSet including any value from `collections` that does
+     * not already exist in this OrderedSet.
+     *
+     * Note: `union` can be used in `withMutations`.
+     * @alias merge
+     * @alias concat
      */
-    concat<C>(...valuesOrCollections: Array<Iterable<C> | C>): OrderedSet<T | C>;
+    union<C>(...collections: Array<Iterable<C>>): OrderedSet<T | C>;
+    merge<C>(...collections: Array<Iterable<C>>): OrderedSet<T | C>;
+    concat<C>(...collections: Array<Iterable<C>>): OrderedSet<T | C>;
+
+    // Sequence algorithms
 
     /**
      * Returns a new Set with values passed through a
@@ -3013,7 +3036,7 @@ declare module Immutable {
        * All entries will be present in the resulting Seq, even if they
        * are duplicates.
        */
-      concat<C>(...valuesOrCollections: Array<Iterable<C> | C>): Seq.Set<T | C>;
+      concat<U>(...collections: Array<Iterable<U>>): Seq.Set<T | U>;
 
       /**
        * Returns a new Seq.Set with values passed through a
@@ -3717,7 +3740,7 @@ declare module Immutable {
       /**
        * Returns a new Collection with other collections concatenated to this one.
        */
-      concat<C>(...valuesOrCollections: Array<Iterable<C> | C>): Collection.Set<T | C>;
+      concat<U>(...collections: Array<Iterable<U>>): Collection.Set<T | U>;
 
       /**
        * Returns a new Collection.Set with values passed through a
diff --git a/type-definitions/immutable.js.flow b/type-definitions/immutable.js.flow
index 1b2d814504..7c431bc2a0 100644
--- a/type-definitions/immutable.js.flow
+++ b/type-definitions/immutable.js.flow
@@ -401,7 +401,7 @@ declare class SetCollection<+T> extends Collection<T, T> {
   @@iterator(): Iterator<T>;
   toSeq(): SetSeq<T>;
 
-  concat<C>(...iters: Array<Iterable<C> | C>): SetCollection<T | C>;
+  concat<U>(...collections: Iterable<U>[]): SetCollection<T | U>;
 
   // `filter`, `map` and `flatMap` cannot be defined further up the hierarchy,
   // because the implementation for `KeyedCollection` allows the value type to
@@ -619,7 +619,7 @@ declare class SetSeq<+T> extends Seq<T, T> mixins SetCollection<T> {
 
   // Override specialized return types
 
-  concat<C>(...iters: Array<Iterable<C> | C>): SetSeq<T | C>;
+  concat<U>(...collections: Iterable<U>[]): SetSeq<T | U>;
 
   filter(predicate: typeof Boolean): SetSeq<$NonMaybeType<T>>;
   filter(
@@ -836,6 +836,9 @@ declare class Map<K, +V> extends KeyedCollection<K, V> {
   merge<K_, V_>(
     ...collections: (Iterable<[K_, V_]> | PlainObjInput<K_, V_>)[]
   ): Map<K | K_, V | V_>;
+  concat<K_, V_>(
+    ...collections: (Iterable<[K_, V_]> | PlainObjInput<K_, V_>)[]
+  ): Map<K | K_, V | V_>;
 
   mergeWith<K_, W, X>(
     merger: (oldVal: V, newVal: W, key: K) => X,
@@ -883,9 +886,6 @@ declare class Map<K, +V> extends KeyedCollection<K, V> {
 
   flip(): Map<V, K>;
 
-  concat<KC, VC>(...iters: Array<Iterable<[KC, VC]>>): Map<K | KC, V | VC>;
-  concat<KC, VC>(...iters: Array<PlainObjInput<KC, VC>>): Map<K | KC, V | VC>;
-
   filter(predicate: typeof Boolean): Map<K, $NonMaybeType<V>>;
   filter(
     predicate: (value: V, key: K, iter: this) => mixed,
@@ -937,6 +937,9 @@ declare class OrderedMap<K, +V> extends Map<K, V> {
   merge<K_, V_>(
     ...collections: (Iterable<[K_, V_]> | PlainObjInput<K_, V_>)[]
   ): OrderedMap<K | K_, V | V_>;
+  concat<K_, V_>(
+    ...collections: (Iterable<[K_, V_]> | PlainObjInput<K_, V_>)[]
+  ): OrderedMap<K | K_, V | V_>;
 
   mergeWith<K_, W, X>(
     merger: (oldVal: V, newVal: W, key: K) => X,
@@ -984,9 +987,6 @@ declare class OrderedMap<K, +V> extends Map<K, V> {
 
   flip(): OrderedMap<V, K>;
 
-  concat<KC, VC>(...iters: Array<Iterable<[KC, VC]>>): OrderedMap<K | KC, V | VC>;
-  concat<KC, VC>(...iters: Array<PlainObjInput<KC, VC>>): OrderedMap<K | KC, V | VC>;
-
   filter(predicate: typeof Boolean): OrderedMap<K, $NonMaybeType<V>>;
   filter(
     predicate: (value: V, key: K, iter: this) => mixed,
@@ -1038,6 +1038,7 @@ declare class Set<+T> extends SetCollection<T> {
   clear(): this;
   union<U>(...collections: Iterable<U>[]): Set<T | U>;
   merge<U>(...collections: Iterable<U>[]): Set<T | U>;
+  concat<U>(...collections: Iterable<U>[]): Set<T | U>;
   intersect<U>(...collections: Iterable<U>[]): Set<T & U>;
   subtract(...collections: Iterable<mixed>[]): this;
 
@@ -1048,8 +1049,6 @@ declare class Set<+T> extends SetCollection<T> {
 
   // Override specialized return types
 
-  concat<C>(...iters: Array<Iterable<C> | C>): Set<T | C>;
-
   filter(predicate: typeof Boolean): Set<$NonMaybeType<T>>;
   filter(
     predicate: (value: T, value: T, iter: this) => mixed,
@@ -1087,10 +1086,9 @@ declare class OrderedSet<+T> extends Set<T> {
   add<U>(value: U): OrderedSet<T | U>;
   union<U>(...collections: Iterable<U>[]): OrderedSet<T | U>;
   merge<U>(...collections: Iterable<U>[]): OrderedSet<T | U>;
+  concat<U>(...collections: Iterable<U>[]): OrderedSet<T | U>;
   intersect<U>(...collections: Iterable<U>[]): OrderedSet<T & U>;
 
-  concat<C>(...iters: Array<Iterable<C> | C>): OrderedSet<T | C>;
-
   filter(predicate: typeof Boolean): OrderedSet<$NonMaybeType<T>>;
   filter(
     predicate: (value: T, value: T, iter: this) => mixed,
diff --git a/type-definitions/ts-tests/map.ts b/type-definitions/ts-tests/map.ts
index 3eef16d976..1a8732aae4 100644
--- a/type-definitions/ts-tests/map.ts
+++ b/type-definitions/ts-tests/map.ts
@@ -252,13 +252,13 @@ import { Map, List } from '../../';
   // $ExpectType Map<string, number>
   Map<string, number>().merge({ a: 1 });
 
-  // $ExpectError
+  // $ExpectType Map<string, number | { b: number; }>
   Map<string, number>().merge({ a: { b: 1 } });
 
   // $ExpectType Map<number, number>
   Map<number, number>().merge(Map<number, number>());
 
-  // $ExpectError
+  // $ExpectType Map<number, string | number>
   Map<number, number>().merge(Map<number, string>());
 
   // $ExpectType Map<number, string | number>
diff --git a/type-definitions/ts-tests/ordered-map.ts b/type-definitions/ts-tests/ordered-map.ts
index 609c882015..5fcc6e2591 100644
--- a/type-definitions/ts-tests/ordered-map.ts
+++ b/type-definitions/ts-tests/ordered-map.ts
@@ -252,13 +252,13 @@ import { OrderedMap, List } from '../../';
   // $ExpectType OrderedMap<string, number>
   OrderedMap<string, number>().merge({ a: 1 });
 
-  // $ExpectError
+  // $ExpectType OrderedMap<string, number | { b: number; }>
   OrderedMap<string, number>().merge({ a: { b: 1 } });
 
   // $ExpectType OrderedMap<number, number>
   OrderedMap<number, number>().merge(OrderedMap<number, number>());
 
-  // $ExpectError
+  // $ExpectType OrderedMap<number, string | number>
   OrderedMap<number, number>().merge(OrderedMap<number, string>());
 
   // $ExpectType OrderedMap<number, string | number>
diff --git a/type-definitions/ts-tests/ordered-set.ts b/type-definitions/ts-tests/ordered-set.ts
index ca0607f371..351cb0d5c6 100644
--- a/type-definitions/ts-tests/ordered-set.ts
+++ b/type-definitions/ts-tests/ordered-set.ts
@@ -155,7 +155,7 @@ import { OrderedSet, Map } from '../../';
   // $ExpectType OrderedSet<number>
   OrderedSet<number>().union(OrderedSet<number>());
 
-  // $ExpectError
+  // $ExpectType OrderedSet<string | number>
   OrderedSet<number>().union(OrderedSet<string>());
 
   // $ExpectType OrderedSet<string | number>
@@ -170,7 +170,7 @@ import { OrderedSet, Map } from '../../';
   // $ExpectType OrderedSet<number>
   OrderedSet<number>().merge(OrderedSet<number>());
 
-  // $ExpectError
+  // $ExpectType OrderedSet<string | number>
   OrderedSet<number>().merge(OrderedSet<string>());
 
   // $ExpectType OrderedSet<string | number>
diff --git a/type-definitions/ts-tests/set.ts b/type-definitions/ts-tests/set.ts
index db120afa4d..6f9a33d8d5 100644
--- a/type-definitions/ts-tests/set.ts
+++ b/type-definitions/ts-tests/set.ts
@@ -155,7 +155,7 @@ import { Set, Map } from '../../';
   // $ExpectType Set<number>
   Set<number>().union(Set<number>());
 
-  // $ExpectError
+  // $ExpectType Set<string | number>
   Set<number>().union(Set<string>());
 
   // $ExpectType Set<string | number>
@@ -170,7 +170,7 @@ import { Set, Map } from '../../';
   // $ExpectType Set<number>
   Set<number>().merge(Set<number>());
 
-  // $ExpectError
+  // $ExpectType Set<string | number>
   Set<number>().merge(Set<string>());
 
   // $ExpectType Set<string | number>