diff --git a/src/Language/GraphQL/AST/Encoder.hs b/src/Language/GraphQL/AST/Encoder.hs
index bd6dab8..ab17929 100644
--- a/src/Language/GraphQL/AST/Encoder.hs
+++ b/src/Language/GraphQL/AST/Encoder.hs
@@ -128,6 +128,13 @@ typeDefinition formatter = \case
         <> optempty (directives formatter) directives'
         <> eitherFormat formatter " " ""
         <> bracesList formatter (fieldDefinition nextFormatter) fields'
+    Full.UnionTypeDefinition description' name' directives' members'
+        -> optempty (description formatter) description'
+        <> "union "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> unionMemberTypes formatter members'
     _typeDefinition' -> "" -- TODO: Types missing.
   where
     nextFormatter = incrementIndent formatter
@@ -136,10 +143,23 @@ implementsInterfaces :: Foldable t => Full.ImplementsInterfaces t -> Lazy.Text
 implementsInterfaces (Full.ImplementsInterfaces interfaces)
     | null interfaces = mempty
     | otherwise = Lazy.Text.fromStrict
-        $ Text.append "implements"
+        $ Text.append "implements "
         $ Text.intercalate " & "
         $ toList interfaces
 
+unionMemberTypes :: Foldable t => Formatter -> Full.UnionMemberTypes t -> Lazy.Text
+unionMemberTypes formatter (Full.UnionMemberTypes memberTypes)
+    | null memberTypes = mempty
+    | Minified <- formatter = Lazy.Text.fromStrict
+        $ Text.append "= "
+        $ Text.intercalate " | "
+        $ toList memberTypes
+    | Pretty _ <- formatter
+        = Lazy.Text.append "="
+        $ Lazy.Text.concat
+        $ (("\n" <> indentSymbol <> "| ") <>) . Lazy.Text.fromStrict
+        <$> toList memberTypes
+
 description :: Formatter -> Full.Description -> Lazy.Text.Text
 description _formatter (Full.Description Nothing) = ""
 description formatter (Full.Description (Just description')) =
diff --git a/tests/Language/GraphQL/AST/EncoderSpec.hs b/tests/Language/GraphQL/AST/EncoderSpec.hs
index 862fa34..5116c0e 100644
--- a/tests/Language/GraphQL/AST/EncoderSpec.hs
+++ b/tests/Language/GraphQL/AST/EncoderSpec.hs
@@ -217,3 +217,15 @@ spec = do
                 |]
                 actual = typeSystemDefinition pretty definition'
              in actual `shouldBe` expected
+
+        it "encodes an enum definition" $
+            let definition' = Full.TypeDefinition
+                    $ Full.UnionTypeDefinition mempty "SearchResult" mempty
+                    $ Full.UnionMemberTypes ["Photo", "Person"]
+                expected = [gql|
+                  union SearchResult =
+                    | Photo
+                    | Person
+                |]
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected