diff --git a/mongoengine/document.py b/mongoengine/document.py
index 06867168..3b812abb 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -105,6 +105,19 @@ class Document(BaseDocument):
 
             if rule == CASCADE:
                 document_cls.objects(**{field_name: self.id}).delete(safe=safe)
+            elif rule == NULLIFY:
+                # TODO: For now, this makes the nullify test pass, but it would
+                # be nicer to use any of these two atomic versions:
+                #
+                #    document_cls.objects(**{field_name: self.id}).update(**{'unset__%s' % field_name: 1})
+                # or
+                #    document_cls.objects(**{field_name: self.id}).update(**{'set__%s' % field_name: None})
+                #
+                # However, I'm getting ValidationError: 1/None is not a valid ObjectId
+                # Anybody got a clue?
+                for doc in document_cls.objects(**{field_name: self.id}):
+                    doc.reviewer = None
+                    doc.save()
 
         id_field = self._meta['id_field']
         object_id = self._fields[id_field].to_mongo(self[id_field])
diff --git a/tests/document.py b/tests/document.py
index d5807c90..7f92320d 100644
--- a/tests/document.py
+++ b/tests/document.py
@@ -625,7 +625,7 @@ class DocumentTest(unittest.TestCase):
         BlogPost.drop_collection()
 
 
-    def test_cascade_delete(self):
+    def test_delete_rule_cascade_and_nullify(self):
         """Ensure that a referenced document is also deleted upon deletion.
         """
 
@@ -633,6 +633,7 @@ class DocumentTest(unittest.TestCase):
             meta = {'collection': 'blogpost_1'}
             content = StringField()
             author = ReferenceField(self.Person, delete_rule=CASCADE)
+            reviewer = ReferenceField(self.Person, delete_rule=NULLIFY)
 
         self.Person.drop_collection()
         BlogPost.drop_collection()
@@ -640,10 +641,18 @@ class DocumentTest(unittest.TestCase):
         author = self.Person(name='Test User')
         author.save()
 
+        reviewer = self.Person(name='Re Viewer')
+        reviewer.save()
+
         post = BlogPost(content = 'Watched some TV')
         post.author = author
+        post.reviewer = reviewer
         post.save()
 
+        reviewer.delete()
+        self.assertEqual(len(BlogPost.objects), 1)  # No effect on the BlogPost
+        self.assertEqual(BlogPost.objects.get().reviewer, None)
+
         # Delete the Person, which should lead to deletion of the BlogPost, too
         author.delete()
         self.assertEqual(len(BlogPost.objects), 0)