diff --git a/lodash.js b/lodash.js
index db04ca13f..b405e83de 100644
--- a/lodash.js
+++ b/lodash.js
@@ -8572,6 +8572,12 @@
         var result = collection.length;
         return (result && isString(collection)) ? stringSize(collection) : result;
       }
+      if (isObjectLike(collection)) {
+        var tag = getTag(collection);
+        if (tag == mapTag || tag == setTag) {
+          return collection.size;
+        }
+      }
       return keys(collection).length;
     }
 
diff --git a/test/test.js b/test/test.js
index 425e23281..4c0c869e6 100644
--- a/test/test.js
+++ b/test/test.js
@@ -19104,6 +19104,34 @@
       assert.strictEqual(_.size(new Foo(array)), 3);
     });
 
+    QUnit.test('should work with maps', function(assert) {
+      assert.expect(1);
+
+      if (Map) {
+        var map = new Map;
+        map.set('a', 1);
+        map.set('b', 2);
+        assert.strictEqual(_.size(map), 2);
+      }
+      else {
+        skipAssert(assert);
+      }
+    });
+
+    QUnit.test('should work with sets', function(assert) {
+      assert.expect(1);
+
+      if (Set) {
+        var set = new Set;
+        set.add(1);
+        set.add(2);
+        assert.strictEqual(_.size(set), 2);
+      }
+      else {
+        skipAssert(assert);
+      }
+    });
+
     QUnit.test('should not treat objects with negative lengths as array-like', function(assert) {
       assert.expect(1);