diff --git a/mongoengine/base/document.py b/mongoengine/base/document.py
index f8ab73d0d..7b2545829 100644
--- a/mongoengine/base/document.py
+++ b/mongoengine/base/document.py
@@ -291,7 +291,7 @@ def get_text_score(self):
 
         return self._data['_text_score']
 
-    def to_mongo(self, use_db_field=True, fields=None):
+    def to_mongo(self, use_db_field=True, fields=None, populate=[]):
         """
         Return as SON data ready for use with MongoDB.
         """
@@ -302,10 +302,19 @@ def to_mongo(self, use_db_field=True, fields=None):
         data['_id'] = None
         data['_cls'] = self._class_name
 
+        Document = _import_class("Document")
+        ReferenceField = _import_class("ReferenceField")
+        ListField = _import_class("ListField")
+
         # only root fields ['test1.a', 'test2'] => ['test1', 'test2']
         root_fields = set([f.split('.')[0] for f in fields])
 
+        if populate:
+            populate_list = [_field.split('.') for _field in populate]
+            populate_domain = dict((_field[0], _field[1:]) for _field in populate_list)
+
         for field_name in self:
+
             if root_fields and field_name not in root_fields:
                 continue
 
@@ -331,6 +340,27 @@ def to_mongo(self, use_db_field=True, fields=None):
 
                 value = field.to_mongo(value, **ex_vars)
 
+                if populate and field_name in populate_domain.keys():
+                    if isinstance(field, ListField):
+                        _obj = []
+                        for ref in value:
+                            if isinstance(ref, ObjectId):
+                                _ref_model = field.field.document_type
+                                _obj.append(_ref_model.objects.get(id=ref).to_mongo(populate=['.'.join(populate_domain[field_name])]))
+
+                            elif isinstance(ref, Document):
+                                _obj.append(ref.to_mongo(populate=['.'.join(populate_domain[field_name])]))
+
+                            else:
+                                _obj.append(ref)
+
+                        value = _obj
+
+                    if isinstance(field, ReferenceField):
+                        _ref_model = field.document_type
+                        _obj = _ref_model.objects.get(id=value).to_mongo(populate=['.'.join(populate_domain[field_name])])
+                        value = _obj
+
             # Handle self generating fields
             if value is None and field._auto_gen:
                 value = field.generate()
@@ -399,9 +429,11 @@ def to_json(self, *args, **kwargs):
         :param use_db_field: Serialize field names as they appear in
             MongoDB (as opposed to attribute names on this document).
             Defaults to True.
+        :param populate:
         """
+        populate = kwargs.pop('populate', [])
         use_db_field = kwargs.pop('use_db_field', True)
-        return json_util.dumps(self.to_mongo(use_db_field), *args, **kwargs)
+        return json_util.dumps(self.to_mongo(use_db_field, populate=populate), *args, **kwargs)
 
     @classmethod
     def from_json(cls, json_data, created=False):
diff --git a/tests/document/json_serialisation.py b/tests/document/json_serialisation.py
index 110f1e14d..bdfa80cf2 100644
--- a/tests/document/json_serialisation.py
+++ b/tests/document/json_serialisation.py
@@ -60,6 +60,29 @@ def __eq__(self, other):
 
         self.assertEqual(doc, Doc.from_json(doc.to_json()))
 
+    def test_populate(self):
+        class VerySimple(Document):
+            num = StringField(default='2')
+
+        class Simple(Document):
+            val = StringField(default='1')
+            inner = ReferenceField(VerySimple)
+
+        class Doc(Document):
+            simple_ref = ReferenceField(Simple)
+            list_ref = ListField(ReferenceField(Simple))
+
+        docum = Doc(simple_ref=Simple(inner=VerySimple().save()).save())
+        docum.list_ref=[Simple(inner=VerySimple().save()).save(), Simple().save()]
+        docum.save()
+        _json = docum.to_json( populate=['simple_ref','list_ref.inner'])
+        
+        import json
+        _json = json.loads(_json)
+        print(_json)
+        self.assertEqual(_json['simple_ref']['val'],'1')
+        self.assertEqual(_json['list_ref'][0]['inner']['num'],'2')
+
     def test_json_complex(self):
 
         if pymongo.version_tuple[0] <= 2 and pymongo.version_tuple[1] <= 3: