diff --git a/omegaconf/dictconfig.py b/omegaconf/dictconfig.py
index 9cb310c84..f9f9aff6c 100644
--- a/omegaconf/dictconfig.py
+++ b/omegaconf/dictconfig.py
@@ -1,7 +1,6 @@
 from .config import Config
+from .errors import ReadonlyConfigError, MissingMandatoryValue
 from .nodes import BaseNode, UntypedNode
-from .errors import ReadonlyConfigError
-import copy
 
 
 class DictConfig(Config):
@@ -98,6 +97,22 @@ def pop(self, key, default=__marker):
     def keys(self):
         return self.content.keys()
 
+    def __contains__(self, key):
+        """
+        A key is contained in a DictConfig if there is an associated value and it is not a mandatory missing value ('???').
+        :param key:
+        :return:
+        """
+        node = self.get_node(key)
+        if node is None:
+            return False
+        else:
+            try:
+                self._resolve_with_default(key, node, None)
+                return True
+            except (MissingMandatoryValue, KeyError):
+                return False
+
     def __iter__(self):
         return iter(self.keys())
 
diff --git a/tests/test_basic_ops_dict.py b/tests/test_basic_ops_dict.py
index f99e03675..c5c154d99 100644
--- a/tests/test_basic_ops_dict.py
+++ b/tests/test_basic_ops_dict.py
@@ -207,15 +207,19 @@ def test_dict_pop():
         c.pop('not_found')
 
 
-def test_in_dict():
-    c = OmegaConf.create(dict(
-        a=1,
-        b=2,
-        c={}))
-    assert 'a' in c
-    assert 'b' in c
-    assert 'c' in c
-    assert 'd' not in c
+@pytest.mark.parametrize("conf,key,expected", [
+    ({"a": 1, "b": {}}, "a", True),
+    ({"a": 1, "b": {}}, "b", True),
+    ({"a": 1, "b": {}}, "c", False),
+    ({"a": 1, "b": "${a}"}, "b", True),
+    ({"a": 1, "b": "???"}, "b", False),
+    ({"a": 1, "b": "???", "c": "${b}"}, "c", False),
+    ({"a": 1, "b": "${not_found}"}, "b", False),
+])
+def test_in_dict(conf, key, expected):
+    conf = OmegaConf.create(conf)
+    ret = key in conf
+    assert ret == expected
 
 
 def test_get_root():