diff --git a/adapter/avro/mu-avro.cabal b/adapter/avro/mu-avro.cabal
index da067a6..11f649b 100644
--- a/adapter/avro/mu-avro.cabal
+++ b/adapter/avro/mu-avro.cabal
@@ -38,7 +38,7 @@ library
     , containers            >=0.6   && <0.7
     , deepseq               >=1.4   && <2
     , language-avro         >=0.1.3 && <0.2
-    , mu-rpc                >=0.4   && <0.5
+    , mu-rpc                >=0.4   && <0.6
     , mu-schema             >=0.3   && <0.4
     , sop-core              >=0.5.0 && <0.6
     , tagged                >=0.8.6 && <0.9
diff --git a/adapter/protobuf/mu-protobuf.cabal b/adapter/protobuf/mu-protobuf.cabal
index 20c8755..edd7b12 100644
--- a/adapter/protobuf/mu-protobuf.cabal
+++ b/adapter/protobuf/mu-protobuf.cabal
@@ -38,7 +38,7 @@ library
     , http-client             >=0.6   && <0.7
     , http2-grpc-proto3-wire  >=0.1   && <0.2
     , language-protobuf       >=1.0.1 && <2
-    , mu-rpc                  >=0.4   && <0.5
+    , mu-rpc                  >=0.4   && <0.6
     , mu-schema               >=0.3   && <0.4
     , proto3-wire             >=1.1   && <2
     , servant-client-core     >=0.16  && <0.19
diff --git a/core/rpc/mu-rpc.cabal b/core/rpc/mu-rpc.cabal
index 5f179b9..3015755 100644
--- a/core/rpc/mu-rpc.cabal
+++ b/core/rpc/mu-rpc.cabal
@@ -1,5 +1,5 @@
 name:               mu-rpc
-version:            0.4.0.1
+version:            0.5.0.0
 synopsis:           Protocol-independent declaration of services and servers.
 description:
   Protocol-independent declaration of services and servers for mu-haskell.
diff --git a/core/rpc/src/Mu/Rpc.hs b/core/rpc/src/Mu/Rpc.hs
index ca38563..a14b5c2 100644
--- a/core/rpc/src/Mu/Rpc.hs
+++ b/core/rpc/src/Mu/Rpc.hs
@@ -20,7 +20,7 @@ and protocol.
 -}
 module Mu.Rpc (
   Package', Package(..)
-, Service', Service(..), Object
+, Service', Service(..), Object, Union
 , Method', Method(..), ObjectField
 , LookupService, LookupMethod
 , TypeRef(..), Argument', Argument(..), Return(..)
@@ -56,6 +56,7 @@ data Package serviceName methodName argName tyRef
 data Service serviceName methodName argName tyRef
   = Service serviceName
             [Method serviceName methodName argName tyRef]
+  | OneOf serviceName [serviceName]
 
 -- | A method is defined by its name, arguments, and return type.
 data Method serviceName methodName argName tyRef
@@ -66,6 +67,8 @@ data Method serviceName methodName argName tyRef
 -- Synonyms for GraphQL
 -- | An object is a set of fields, in GraphQL lingo.
 type Object = 'Service
+-- | A union is one of the objects.
+type Union = 'OneOf
 -- | A field in an object takes some input objects,
 --   and returns a value or some other object,
 --   in GraphQL lingo.
@@ -76,6 +79,7 @@ type family LookupService (ss :: [Service snm mnm anm tr]) (s :: snm)
               :: Service snm mnm anm tr where
   LookupService '[] s = TypeError ('Text "could not find method " ':<>: 'ShowType s)
   LookupService ('Service s ms ': ss) s = 'Service s ms
+  LookupService ('OneOf   s ms ': ss) s = 'OneOf   s ms
   LookupService (other         ': ss) s = LookupService ss s
 
 -- | Look up a method in a service definition using its name.
@@ -136,7 +140,7 @@ data RpcInfo i
   = NoRpcInfo
   | RpcInfo { packageInfo :: Package Text Text Text TyInfo
             , serviceInfo :: Service Text Text Text TyInfo
-            , methodInfo  :: Method  Text Text Text TyInfo
+            , methodInfo  :: Maybe (Method Text Text Text TyInfo)
             , headers     :: RequestHeaders
             , extraInfo   :: i
             }
@@ -150,10 +154,15 @@ data TyInfo
 instance Show (RpcInfo i) where
   show NoRpcInfo
     = "<no info>"
-  show (RpcInfo (Package Nothing _) (Service s _) (Method m _ _) _ _)
-    = T.unpack (s <> ":" <> m)
-  show (RpcInfo (Package (Just p) _) (Service s _) (Method m _ _) _ _)
-    = T.unpack (p <> ":" <> s <> ":" <> m)
+  show (RpcInfo (Package p _) s m _ _)
+    = T.unpack $ showPkg p (showMth m (showSvc s))
+    where
+      showPkg Nothing    = id
+      showPkg (Just pkg) = ((pkg <> ":") <>)
+      showMth Nothing                = id
+      showMth (Just (Method mt _ _)) = (<> (":" <> mt))
+      showSvc (Service sv _) = sv
+      showSvc (OneOf   sv _) = sv
 
 class ReflectRpcInfo (p :: Package') (s :: Service') (m :: Method') where
   reflectRpcInfo :: Proxy p -> Proxy s -> Proxy m -> RequestHeaders -> i -> RpcInfo i
@@ -175,6 +184,13 @@ instance KnownMaySymbol 'Nothing where
 instance (KnownSymbol s) => KnownMaySymbol ('Just s) where
   maySymbolVal _ = Just $ T.pack $ symbolVal (Proxy @s)
 
+class KnownSymbols (m :: [Symbol]) where
+  symbolsVal :: Proxy m -> [Text]
+instance KnownSymbols '[] where
+  symbolsVal _ = []
+instance (KnownSymbol s, KnownSymbols ss) => KnownSymbols (s ': ss) where
+  symbolsVal _ = T.pack (symbolVal (Proxy @s)) : symbolsVal (Proxy @ss)
+
 class ReflectServices (ss :: [Service']) where
   reflectServices :: Proxy ss -> [Service Text Text Text TyInfo]
 instance ReflectServices '[] where
@@ -204,7 +220,7 @@ instance (KnownMaySymbol pname, ReflectServices ss, ReflectService s, ReflectMet
   reflectRpcInfo _ ps pm req extra
     = RpcInfo (Package (maySymbolVal (Proxy @pname))
                        (reflectServices (Proxy @ss)))
-              (reflectService ps) (reflectMethod pm) req extra
+              (reflectService ps) (Just (reflectMethod pm)) req extra
 
 instance (KnownSymbol sname, ReflectMethods ms)
          => ReflectService ('Service sname ms) where
@@ -212,6 +228,12 @@ instance (KnownSymbol sname, ReflectMethods ms)
     = Service (T.pack $ symbolVal (Proxy @sname))
               (reflectMethods (Proxy @ms))
 
+instance (KnownSymbol sname, KnownSymbols elts)
+         => ReflectService ('OneOf sname elts) where
+  reflectService _
+    = OneOf (T.pack $ symbolVal (Proxy @sname))
+            (symbolsVal (Proxy @elts))
+
 instance (KnownSymbol mname, ReflectArgs args, ReflectReturn r)
          => ReflectMethod ('Method mname args r) where
   reflectMethod _
diff --git a/core/rpc/src/Mu/Rpc/Examples.hs b/core/rpc/src/Mu/Rpc/Examples.hs
index cf3c608..33c7552 100644
--- a/core/rpc/src/Mu/Rpc/Examples.hs
+++ b/core/rpc/src/Mu/Rpc/Examples.hs
@@ -105,30 +105,43 @@ quickstartServer
 type ApolloService
   = 'Package ('Just "apollo")
       '[ Object "Book"
-        '[ ObjectField "title" '[] ('RetSingle ('PrimitiveRef String))
-        , ObjectField "author" '[] ('RetSingle ('ObjectRef "Author"))
-        ]
-      , Object "Author"
-        '[ ObjectField "name" '[] ('RetSingle ('PrimitiveRef String))
-        , ObjectField "books" '[] ('RetSingle ('ListRef ('ObjectRef "Book")))
-        ]
+         '[ ObjectField "title" '[] ('RetSingle ('PrimitiveRef String))
+          , ObjectField "author" '[] ('RetSingle ('ObjectRef "Author"))
+          ]
+      ,  Object "Paper"
+         '[ ObjectField "title" '[] ('RetSingle ('PrimitiveRef String))
+          , ObjectField "author" '[] ('RetSingle ('ObjectRef "Author"))
+          ]
+      ,  Union "Writing" ["Book", "Paper"]
+      ,  Object "Author"
+         '[ ObjectField "name" '[] ('RetSingle ('PrimitiveRef String))
+          , ObjectField "writings" '[] ('RetSingle ('ListRef ('ObjectRef "Writing")))
+          ]
       ]
 
 type ApolloBookAuthor = '[
-    "Book"   ':-> (String, Integer)
-  , "Author" ':-> Integer
+    "Book"    ':-> (String, Integer)
+  , "Paper"   ':-> (String, Integer)
+  , "Writing" ':-> Either (String, Integer) (String, Integer)
+  , "Author"  ':-> Integer
   ]
 
 apolloServer :: forall m i. (MonadServer m)
              => ServerT ApolloBookAuthor i ApolloService m _
 apolloServer
   = resolver
-      ( object @"Author" ( field @"name"   authorName
-                         , field @"books"  authorBooks )
+      ( object @"Author" ( field @"name"     authorName
+                         , field @"writings" authorWrs )
       , object @"Book"   ( field @"author" (pure . snd)
-                         , field @"title"  (pure . fst) ) )
+                         , field @"title"  (pure . fst) )
+      , object @"Paper"  ( field @"author" (pure . snd)
+                         , field @"title"  (pure . fst) )
+      , union @"Writing" writing )
   where
     authorName :: Integer -> m String
     authorName _ = pure "alex"  -- this would run in the DB
-    authorBooks :: Integer -> m [(String, Integer)]
-    authorBooks _ = pure []
+    authorWrs :: Integer -> m [Either (String, Integer) (String, Integer)]
+    authorWrs _ = pure []
+
+    writing (Left  c) = pure $ unionChoice @"Book" c
+    writing (Right c) = pure $ unionChoice @"Paper" c
diff --git a/core/rpc/src/Mu/Server.hs b/core/rpc/src/Mu/Server.hs
index e862b7c..ca7922b 100644
--- a/core/rpc/src/Mu/Server.hs
+++ b/core/rpc/src/Mu/Server.hs
@@ -1,3 +1,4 @@
+{-# language AllowAmbiguousTypes       #-}
 {-# language CPP                       #-}
 {-# language ConstraintKinds           #-}
 {-# language DataKinds                 #-}
@@ -54,12 +55,13 @@ module Mu.Server (
   -- ** Definitions by name
 , singleService
 , method, methodWithInfo
-, resolver, object
+, resolver, object, union
 , field, fieldWithInfo
+, UnionChoice(..), unionChoice
 , NamedList(..)
   -- ** Definitions by position
 , SingleServerT, pattern Server
-, ServerT(..), ServicesT(..), HandlersT(.., (:<||>:), (:<|>:))
+, ServerT(..), ServicesT(..), ServiceT(..), HandlersT(.., (:<||>:), (:<|>:))
   -- ** Simple servers using only IO
 , ServerErrorIO, ServerIO
   -- * Errors which might be raised
@@ -74,6 +76,7 @@ import           Control.Exception    (Exception)
 import           Control.Monad.Except
 import           Data.Conduit
 import           Data.Kind
+import           Data.Typeable
 import           GHC.TypeLits
 
 import           Mu.Rpc
@@ -151,7 +154,7 @@ data ServerT (chn :: ServiceChain snm) (info :: Type)
 pattern Server :: (MappingRight chn sname ~ ())
                => HandlersT chn info () methods m hs
                -> ServerT chn info ('Package pname '[ 'Service sname methods ]) m '[hs]
-pattern Server svr = Services (svr :<&>: S0)
+pattern Server svr = Services (ProperSvc svr :<&>: S0)
 
 infixr 3 :<&>:
 -- | Definition of a complete server for a service.
@@ -159,9 +162,31 @@ data ServicesT (chn :: ServiceChain snm) (info :: Type)
                (s :: [Service snm mnm anm (TypeRef snm)])
                (m :: Type -> Type) (hs :: [[Type]]) where
   S0 :: ServicesT chn info '[] m '[]
-  (:<&>:) :: HandlersT chn info (MappingRight chn sname) methods m hs
+  (:<&>:) :: ServiceT chn info svc m hs
           -> ServicesT chn info rest m hss
-          -> ServicesT chn info ('Service sname methods ': rest) m (hs ': hss)
+          -> ServicesT chn info (svc ': rest) m (hs ': hss)
+
+type family InUnion (x :: k) (xs :: [k]) :: Constraint where
+  InUnion x '[] = TypeError ('ShowType x ':<>: 'Text " is not part of the union")
+  InUnion x (x ': xs) = ()
+  InUnion x (y ': xs) = InUnion x xs
+
+data UnionChoice chn elts where
+  UnionChoice :: (InUnion elt elts, Typeable elt)
+              => Proxy elt -> MappingRight chn elt
+              -> UnionChoice chn elts
+
+unionChoice :: forall elt elts chn.
+               (InUnion elt elts, Typeable elt)
+            => MappingRight chn elt -> UnionChoice chn elts
+unionChoice = UnionChoice (Proxy @elt)
+
+-- | Definition of different kinds of services.
+data ServiceT chn info svc m hs where
+  ProperSvc :: HandlersT chn info (MappingRight chn sname) methods m hs
+            -> ServiceT chn info ('Service sname methods) m hs
+  OneOfSvc  :: (MappingRight chn sname -> m (UnionChoice chn elts))
+            -> ServiceT chn info ('OneOf sname elts) m '[]
 
 -- | 'HandlersT' is a sequence of handlers.
 --   Note that the handlers for your service
@@ -322,6 +347,11 @@ object
   => p -> Named sname (HandlersT chn info (MappingRight chn sname) ms m hs)
 object nl = Named $ toHandlers $ toNamedList nl
 
+union :: forall sname chn m elts.
+         (MappingRight chn sname -> m (UnionChoice chn elts))
+      -> Named sname (MappingRight chn sname -> m (UnionChoice chn elts))
+union = Named
+
 -- | Combines the implementation of several GraphQL objects,
 --   which means a whole Mu service for a GraphQL server.
 --   Intented to be used with a tuple of 'objects':
@@ -412,7 +442,12 @@ instance ToServices chn info '[] m '[] nl where
 instance ( FindService name (HandlersT chn info (MappingRight chn name) methods m h) nl
          , ToServices chn info ss m hs nl)
          => ToServices chn info ('Service name methods ': ss) m (h ': hs) nl where
-  toServices nl = findService (Proxy @name) nl :<&>: toServices nl
+  toServices nl = ProperSvc (findService (Proxy @name) nl) :<&>: toServices nl
+instance ( FindService name (MappingRight chn name -> m (UnionChoice chn elts)) nl
+         , ToServices chn info ss m hs nl)
+         => ToServices chn info ('OneOf name elts ': ss) m ('[] ': hs) nl where
+  toServices nl = OneOfSvc (findService (Proxy @name) nl) :<&>: toServices nl
+
 
 class FindService name h nl | name nl -> h where
   findService :: Proxy name -> NamedList nl -> h
diff --git a/examples/library b/examples/library
index 851f952..b2cfb27 160000
--- a/examples/library
+++ b/examples/library
@@ -1 +1 @@
-Subproject commit 851f9522792e843f2e4b5e478cae6714b5cefd79
+Subproject commit b2cfb27c24d14fe51c0d7d13b602eb7584a9a598
diff --git a/graphql/exe/Main.hs b/graphql/exe/Main.hs
index b1fb95a..7d8fd09 100644
--- a/graphql/exe/Main.hs
+++ b/graphql/exe/Main.hs
@@ -1,6 +1,7 @@
 {-# language CPP                   #-}
 {-# language DataKinds             #-}
 {-# language FlexibleContexts      #-}
+{-# language LambdaCase            #-}
 {-# language OverloadedStrings     #-}
 {-# language PartialTypeSignatures #-}
 {-# language PolyKinds             #-}
@@ -52,9 +53,14 @@ main = do
     (Proxy @'Nothing)
     (Proxy @('Just "Subscription"))
 
+data WritingMapping
+  = ABook (Integer, Integer) | AnArticle (Integer, Integer)
+
 type ServiceMapping = '[
-    "Book"   ':-> (Integer, Integer)
-  , "Author" ':-> Integer
+    "Book"    ':-> (Integer, Integer)
+  , "Article" ':-> (Integer, Integer)
+  , "Author"  ':-> Integer
+  , "Writing" ':-> WritingMapping
   ]
 
 library :: [(Integer, T.Text, [(Integer, (T.Text, Integer))])]
@@ -64,39 +70,54 @@ library
     , (3, "Michael Ende", [(4, ("The Neverending Story", 5)), (5, ("Momo", 3))])
     ]
 
+articles :: [(Integer, T.Text, [(Integer, (T.Text, Integer))])]
+articles
+  = [ (1, "Fuencislo Robles", [(6, ("On Warm Chocolate", 4)), (2, ("On Cold Chocolate", 4))]) ]
+
 libraryServer :: forall m i. (MonadServer m)
               => ServerT ServiceMapping i ServiceDefinition m _
 libraryServer
-  = resolver ( object @"Book"   ( field  @"id"      bookId
+  = resolver ( object @"Book"   ( field  @"id"      bookOrArticleId
                                 , field  @"title"   bookTitle
-                                , field  @"author"  bookAuthor
+                                , field  @"author"  bookOrArticleAuthor
                                 , field  @"info"    bookInfo )
+             , object @"Article" ( field @"id"      bookOrArticleId
+                                , field  @"title"   articleTitle
+                                , field  @"author"  bookOrArticleAuthor )
              , object @"Author" ( field  @"id"      authorId
                                 , field  @"name"    authorName
-                                , field  @"books"   authorBooks )
+                                , field  @"writings" authorBooks )
              , object @"Query"  ( method @"author"  findAuthor
                                 , method @"book"    findBookTitle
                                 , method @"authors" allAuthors
                                 , method @"books"   allBooks' )
              , object @"Subscription" ( method @"books" allBooksConduit )
+             , union @"Writing" (\case (ABook     x) -> pure $ unionChoice @"Book"    x
+                                       (AnArticle x) -> pure $ unionChoice @"Article" x)
              )
   where
     findBook i = find ((==i) . fst3) library
+    findArticle i = find ((==i) . fst3) articles
 
-    bookId (_, bid) = pure bid
+    bookOrArticleId (_, bid) = pure bid
+    bookOrArticleAuthor (aid, _) = pure aid
     bookTitle (aid, bid) = pure $ fromMaybe "" $ do
       bk <- findBook aid
       ev <- lookup bid (thd3 bk)
       pure (fst ev)
-    bookAuthor (aid, _) = pure aid
     bookInfo (aid, bid) = pure $ do
       bk <- findBook aid
       ev <- lookup bid (thd3 bk)
       pure $ JSON.object ["score" JSON..= snd ev]
+    articleTitle (aid, bid) = pure $ fromMaybe "" $ do
+      bk <- findArticle aid
+      ev <- lookup bid (thd3 bk)
+      pure (fst ev)
 
     authorId = pure
     authorName aid = pure $ maybe "" snd3 (findBook aid)
-    authorBooks aid = pure $ maybe [] (map ((aid,) . fst) . thd3) (findBook aid)
+    authorBooks aid = pure $ maybe [] (map (ABook . (aid,) . fst) . thd3) (findBook aid)
+                           <> maybe [] (map (AnArticle . (aid,) . fst) . thd3) (findArticle aid)
 
     findAuthor rx = pure $ listToMaybe
       [aid | (aid, name, _) <- library, name =~ rx]
diff --git a/graphql/exe/schema.graphql b/graphql/exe/schema.graphql
index 73bd0f7..9d83294 100644
--- a/graphql/exe/schema.graphql
+++ b/graphql/exe/schema.graphql
@@ -5,10 +5,18 @@ type Book {
   info: JSON
 }
 
+type Article {
+  id: Int!
+  title: String!
+  author: Author!
+}
+
+union Writing = Book | Article
+
 type Author {
   id: Int!
   name: String!
-  books: [Book!]!
+  writings: [Writing!]!
 }
 
 type Query {
diff --git a/graphql/mu-graphql.cabal b/graphql/mu-graphql.cabal
index bae35ee..fed2efc 100644
--- a/graphql/mu-graphql.cabal
+++ b/graphql/mu-graphql.cabal
@@ -1,5 +1,5 @@
 name:          mu-graphql
-version:       0.4.1.1
+version:       0.5.0.0
 synopsis:      GraphQL support for Mu
 description:   GraphQL servers and clients for Mu-Haskell
 cabal-version: >=1.10
@@ -40,7 +40,7 @@ library
     , list-t                >=1.0   && <2
     , megaparsec            >=8     && <10
     , mtl                   >=2.2   && <2.3
-    , mu-rpc                >=0.4   && <0.5
+    , mu-rpc                >=0.5   && <0.6
     , mu-schema             >=0.3   && <0.4
     , parsers               >=0.12  && <0.13
     , scientific            >=0.3   && <0.4
@@ -73,7 +73,7 @@ executable library-graphql
     , aeson       >=1.4   && <2
     , conduit     >=1.3.2 && <1.4
     , mu-graphql
-    , mu-rpc      >=0.4   && <0.5
+    , mu-rpc      >=0.5   && <0.6
     , mu-schema   >=0.3   && <0.4
     , regex-tdfa  >=1.3   && <2
     , text        >=1.2   && <2
diff --git a/graphql/src/Mu/GraphQL/Quasi.hs b/graphql/src/Mu/GraphQL/Quasi.hs
index 3125203..372da2d 100644
--- a/graphql/src/Mu/GraphQL/Quasi.hs
+++ b/graphql/src/Mu/GraphQL/Quasi.hs
@@ -61,7 +61,8 @@ data GQLType =
   | Object
   | Scalar
   | InputObject
-  | Other
+  | Union
+  | Interface
 
 classifySchema :: [GQL.TypeSystemDefinition] -> SchemaMap
 classifySchema = foldl' schemaToMap HM.empty
@@ -81,9 +82,9 @@ classify = HM.fromList . (typeToKeyValue <$>)
     typeToKeyValue (GQL.ObjectTypeDefinition _ name _ _ _)
       = (name, Object)
     typeToKeyValue (GQL.InterfaceTypeDefinition _ name _ _)
-      = (name, Other)
+      = (name, Interface)
     typeToKeyValue (GQL.UnionTypeDefinition _ name _ _)
-      = (name, Other)
+      = (name, Union)
     typeToKeyValue (GQL.EnumTypeDefinition _ name _ _)
       = (name, Enum)
     typeToKeyValue (GQL.InputObjectTypeDefinition _ name _ _)
@@ -118,8 +119,11 @@ defaultDeclToTy (sn, (mn, (an, dv)))
 typeToDec :: Name -> TypeMap -> SchemaMap -> GQL.TypeDefinition -> Q Result
 typeToDec _ _ _ GQL.InterfaceTypeDefinition {}
   = fail "interface types are not supported"
-typeToDec _ _ _ GQL.UnionTypeDefinition {}
-  = fail "union types are not supported"
+typeToDec _ _ _ (GQL.UnionTypeDefinition _ nm _ (GQL.UnionMemberTypes elts)) = do
+  selts <- mapM textToStrLit elts
+  GQLService <$> [t| 'OneOf $(textToStrLit nm)
+                            $(pure $ typesToList selts) |]
+             <*> pure []
 typeToDec schemaName tm _ (GQL.ScalarTypeDefinition _ s _) =
   GQLScalar <$ gqlTypeToType s tm schemaName
 typeToDec schemaName tm sm (GQL.ObjectTypeDefinition _ nm _ _ flds) = do
diff --git a/graphql/src/Mu/GraphQL/Query/Definition.hs b/graphql/src/Mu/GraphQL/Query/Definition.hs
index d859180..a8a139c 100644
--- a/graphql/src/Mu/GraphQL/Query/Definition.hs
+++ b/graphql/src/Mu/GraphQL/Query/Definition.hs
@@ -8,6 +8,7 @@ module Mu.GraphQL.Query.Definition where
 import           Data.SOP.NP
 import           Data.SOP.NS
 import           Data.Text
+import           Data.Typeable
 import qualified Language.GraphQL.AST as GQL
 import           Mu.Rpc
 import           Mu.Schema
@@ -27,9 +28,12 @@ data Document (p :: Package snm mnm anm (TypeRef snm))
     => OneMethodQuery ('Package pname ss) (LookupService ss sub)
     -> Document ('Package pname ss) qr mut ('Just sub)
 
-type ServiceQuery (p :: Package snm mnm anm (TypeRef snm))
-                  (s :: Service snm mnm anm (TypeRef snm))
-  = [OneMethodQuery p s]
+data ServiceQuery (p :: Package snm mnm anm (TypeRef snm))
+                  (s :: Service snm mnm anm (TypeRef snm)) where
+  ServiceQuery :: [OneMethodQuery p ('Service nm ms)]
+               -> ServiceQuery p ('Service nm ms)
+  OneOfQuery   :: NP (ChosenOneOfQuery p) elts
+               -> ServiceQuery p ('OneOf nm elts)
 
 data OneMethodQuery (p :: Package snm mnm anm (TypeRef snm))
                     (s :: Service snm mnm anm (TypeRef snm)) where
@@ -40,17 +44,23 @@ data OneMethodQuery (p :: Package snm mnm anm (TypeRef snm))
   -- the special '__typename' field
   TypeNameQuery
     :: Maybe Text
-    -> OneMethodQuery p ('Service nm ms)
+    -> OneMethodQuery p s
   -- introspection fields
   SchemaQuery
     :: Maybe Text
     -> [GQL.Selection]
-    -> OneMethodQuery p ('Service nm ms)
+    -> OneMethodQuery p s
   TypeQuery
     :: Maybe Text
     -> Text
     -> [GQL.Selection]
-    -> OneMethodQuery p ('Service nm ms)
+    -> OneMethodQuery p s
+
+data ChosenOneOfQuery p elt where
+  ChosenOneOfQuery
+    :: Typeable elt => Proxy elt
+    -> ServiceQuery ('Package pname ss) (LookupService ss elt)
+    -> ChosenOneOfQuery ('Package pname ss) elt
 
 data ChosenMethodQuery (p :: Package snm mnm anm (TypeRef snm))
                        (m :: Method snm mnm anm (TypeRef snm)) where
diff --git a/graphql/src/Mu/GraphQL/Query/Introspection.hs b/graphql/src/Mu/GraphQL/Query/Introspection.hs
index 3b39aa8..c11ed14 100644
--- a/graphql/src/Mu/GraphQL/Query/Introspection.hs
+++ b/graphql/src/Mu/GraphQL/Query/Introspection.hs
@@ -33,11 +33,12 @@ data Schema
 
 data Type
   = Type
-    { kind       :: TypeKind
-    , typeName   :: Maybe T.Text
-    , fields     :: [Field]
-    , enumValues :: [EnumValue]
-    , ofType     :: Maybe Type
+    { kind          :: TypeKind
+    , typeName      :: Maybe T.Text
+    , fields        :: [Field]
+    , enumValues    :: [EnumValue]
+    , possibleTypes :: [Type]
+    , ofType        :: Maybe Type
     }
   | TypeRef { to :: T.Text }
   deriving Show
@@ -74,17 +75,17 @@ data TypeKind
   deriving Show
 
 tSimple :: T.Text -> Type
-tSimple t = Type SCALAR (Just t) [] [] Nothing
+tSimple t = Type SCALAR (Just t) [] [] [] Nothing
 
 tList :: Type -> Type
-tList = Type LIST Nothing [] [] . Just
+tList = Type LIST Nothing [] [] [] . Just
 
 tNonNull :: Type -> Type
-tNonNull = Type NON_NULL Nothing [] [] . Just
+tNonNull = Type NON_NULL Nothing [] [] [] . Just
 
 unwrapNonNull :: Type -> Maybe Type
-unwrapNonNull (Type NON_NULL _ _ _ x) = x
-unwrapNonNull _                       = Nothing
+unwrapNonNull (Type NON_NULL _ _ _ _ x) = x
+unwrapNonNull _                         = Nothing
 
 -- BUILD INTROSPECTION DATA
 -- ========================
@@ -175,12 +176,32 @@ instance ( KnownSymbol sname
   introspectServices _ psub = do
     let name = T.pack $ symbolVal (Proxy @sname)
     fs <- introspectFields (Proxy @smethods) (Proxy @(IsSub sname sub))
-    let t = Type OBJECT (Just name) fs [] Nothing
+    let t = Type OBJECT (Just name) fs [] [] Nothing
     -- add this one to the mix
     tell (HM.singleton name t)
     -- continue with the rest
     introspectServices (Proxy @ss) psub
 
+instance ( KnownSymbol sname, KnownSymbols elts
+         , IntrospectServices ss sub )
+         => IntrospectServices ('OneOf sname elts ': ss) sub where
+  introspectServices _ psub = do
+    let name = T.pack $ symbolVal (Proxy @sname)
+        tys  = map tSimple (symbolsVal (Proxy @elts))
+        t    = Type UNION (Just name) [] [] tys Nothing
+    -- add this one to the mix
+    tell (HM.singleton name t)
+    -- continue with the rest
+    introspectServices (Proxy @ss) psub
+
+class KnownSymbols (ss :: [Symbol]) where
+  symbolsVal :: Proxy ss -> [T.Text]
+instance KnownSymbols '[] where
+  symbolsVal _ = []
+instance (KnownSymbol s, KnownSymbols ss)
+         => KnownSymbols (s ': ss) where
+  symbolsVal _ = T.pack (symbolVal (Proxy @s)) : symbolsVal (Proxy @ss)
+
 class IntrospectFields (fs :: [Method']) (isSub :: Bool) where
   introspectFields
     :: Proxy fs -> Proxy isSub -> Writer TypeMap [Field]
@@ -292,7 +313,7 @@ instance (KnownSymbol name, IntrospectSchemaFields fields, IntrospectSchema ts)
   introspectSchema k suffix _ = do
     let name = T.pack (symbolVal (Proxy @name)) <> suffix
         fs   = introspectSchemaFields suffix (Proxy @fields)
-        t    = Type k (Just name) fs [] Nothing
+        t    = Type k (Just name) fs [] [] Nothing
     -- add this one to the mix
     tell (HM.singleton name t)
     -- continue with the rest
@@ -302,7 +323,7 @@ instance (KnownSymbol name, IntrospectSchemaEnum choices, IntrospectSchema ts)
   introspectSchema k suffix _ = do
     let name = T.pack (symbolVal (Proxy @name)) <> suffix
         cs   = introspectSchemaEnum (Proxy @choices)
-        t    = Type ENUM (Just name) [] cs Nothing
+        t    = Type ENUM (Just name) [] cs [] Nothing
     -- add this one to the mix
     tell (HM.singleton name t)
     -- continue with the rest
diff --git a/graphql/src/Mu/GraphQL/Query/Parse.hs b/graphql/src/Mu/GraphQL/Query/Parse.hs
index 15f9a28..3ecf98a 100644
--- a/graphql/src/Mu/GraphQL/Query/Parse.hs
+++ b/graphql/src/Mu/GraphQL/Query/Parse.hs
@@ -22,8 +22,8 @@ import           Data.Int                    (Int32)
 import           Data.List                   (find)
 import           Data.Maybe
 import           Data.Proxy
-import           Data.Scientific             (Scientific, floatingOrInteger, fromFloatDigits)
 import           Data.SOP.NS
+import           Data.Scientific             (Scientific, floatingOrInteger, fromFloatDigits)
 import qualified Data.Text                   as T
 import           GHC.TypeLits
 import qualified Language.GraphQL.AST        as GQL
@@ -139,14 +139,15 @@ instance
     KnownName sub, ParseMethod p ('Service sub smethods) smethods
   ) => ParseTypedDoc p ('Just qr) ('Just mut) ('Just sub) where
   parseTypedDocQuery vmap frmap sset
-    = QueryDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = QueryDoc <$> parseQuery (Proxy @p) (Proxy @qr) vmap frmap sset
   parseTypedDocMutation vmap frmap sset
-    = MutationDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = MutationDoc <$> parseQuery (Proxy @p) (Proxy @mut) vmap frmap sset
   parseTypedDocSubscription vmap frmap sset
-    = do q <- parseQuery Proxy Proxy vmap frmap sset
+    = do q <- parseQuery (Proxy @p) (Proxy @sub) vmap frmap sset
          case q of
-           [one] -> pure $ SubscriptionDoc one
-           _     -> throwError "subscriptions may only have one field"
+           ServiceQuery [one]
+             -> pure $ SubscriptionDoc one
+           _ -> throwError "subscriptions may only have one field"
 
 instance
   ( p ~ 'Package pname ss,
@@ -156,9 +157,9 @@ instance
     KnownName mut, ParseMethod p ('Service mut mmethods) mmethods
   ) => ParseTypedDoc p ('Just qr) ('Just mut) 'Nothing where
   parseTypedDocQuery vmap frmap sset
-    = QueryDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = QueryDoc <$> parseQuery (Proxy @p) (Proxy @qr) vmap frmap sset
   parseTypedDocMutation vmap frmap sset
-    = MutationDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = MutationDoc <$> parseQuery (Proxy @p) (Proxy @mut) vmap frmap sset
   parseTypedDocSubscription _ _ _
     = throwError "no subscriptions are defined in the schema"
 
@@ -170,14 +171,15 @@ instance
     KnownName sub, ParseMethod p ('Service sub smethods) smethods
   ) => ParseTypedDoc p ('Just qr) 'Nothing ('Just sub) where
   parseTypedDocQuery vmap frmap sset
-    = QueryDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = QueryDoc <$> parseQuery (Proxy @p) (Proxy @qr) vmap frmap sset
   parseTypedDocMutation _ _ _
     = throwError "no mutations are defined in the schema"
   parseTypedDocSubscription vmap frmap sset
-    = do q <- parseQuery Proxy Proxy vmap frmap sset
+    = do q <- parseQuery (Proxy @p) (Proxy @sub) vmap frmap sset
          case q of
-           [one] -> pure $ SubscriptionDoc one
-           _     -> throwError "subscriptions may only have one field"
+           ServiceQuery [one]
+             -> pure $ SubscriptionDoc one
+           _ -> throwError "subscriptions may only have one field"
 
 instance
   ( p ~ 'Package pname ss,
@@ -185,7 +187,7 @@ instance
     KnownName qr, ParseMethod p ('Service qr qmethods) qmethods
   ) => ParseTypedDoc p ('Just qr) 'Nothing 'Nothing where
   parseTypedDocQuery vmap frmap sset
-    = QueryDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = QueryDoc <$> parseQuery (Proxy @p) (Proxy @qr) vmap frmap sset
   parseTypedDocMutation _ _ _
     = throwError "no mutations are defined in the schema"
   parseTypedDocSubscription _ _ _
@@ -201,12 +203,13 @@ instance
   parseTypedDocQuery _ _ _
     = throwError "no queries are defined in the schema"
   parseTypedDocMutation vmap frmap sset
-    = MutationDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = MutationDoc <$> parseQuery (Proxy @p) (Proxy @mut) vmap frmap sset
   parseTypedDocSubscription vmap frmap sset
-    = do q <- parseQuery Proxy Proxy vmap frmap sset
+    = do q <- parseQuery (Proxy @p) (Proxy @sub) vmap frmap sset
          case q of
-           [one] -> pure $ SubscriptionDoc one
-           _     -> throwError "subscriptions may only have one field"
+           ServiceQuery [one]
+             -> pure $ SubscriptionDoc one
+           _ -> throwError "subscriptions may only have one field"
 
 instance
   ( p ~ 'Package pname ss,
@@ -216,7 +219,7 @@ instance
   parseTypedDocQuery _ _ _
     = throwError "no queries are defined in the schema"
   parseTypedDocMutation vmap frmap sset
-    = MutationDoc <$> parseQuery Proxy Proxy vmap frmap sset
+    = MutationDoc <$> parseQuery (Proxy @p) (Proxy @mut) vmap frmap sset
   parseTypedDocSubscription _ _ _
     = throwError "no subscriptions are defined in the schema"
 
@@ -230,10 +233,11 @@ instance
   parseTypedDocMutation _ _ _
     = throwError "no mutations are defined in the schema"
   parseTypedDocSubscription vmap frmap sset
-    = do q <- parseQuery Proxy Proxy vmap frmap sset
+    = do q <- parseQuery (Proxy @p) (Proxy @sub) vmap frmap sset
          case q of
-           [one] -> pure $ SubscriptionDoc one
-           _     -> throwError "subscriptions may only have one field"
+           ServiceQuery [one]
+             -> pure $ SubscriptionDoc one
+           _ -> throwError "subscriptions may only have one field"
 
 instance
   ParseTypedDoc p 'Nothing 'Nothing 'Nothing where
@@ -263,59 +267,91 @@ constToValue (GQL.ConstObject n)
       [ GQL.ObjectField a (GQL.Node (constToValue v) m) l
       | GQL.ObjectField a (GQL.Node v m) l <- n ]
 
+class ParseQuery (p :: Package') (s :: Symbol) where
+  parseQuery
+    :: ( MonadError T.Text f, p ~ 'Package pname ss )
+    => Proxy p -> Proxy s
+    -> VariableMap -> FragmentMap -> [GQL.Selection]
+    -> f (ServiceQuery p (LookupService ss s))
 
-parseQuery ::
-  forall (p :: Package') (s :: Symbol) pname ss methods f.
-  ( MonadError T.Text f, p ~ 'Package pname ss,
-    LookupService ss s ~ 'Service s methods,
-    KnownName s, ParseMethod p ('Service s methods) methods
-  ) =>
-  Proxy p ->
-  Proxy s ->
-  VariableMap -> FragmentMap -> [GQL.Selection] ->
-  f (ServiceQuery p (LookupService ss s))
-parseQuery _ _ _ _ [] = pure []
-parseQuery pp ps vmap frmap (GQL.FieldSelection fld : ss)
-  = (++) <$> (maybeToList <$> fieldToMethod fld)
-         <*> parseQuery pp ps vmap frmap ss
-  where
-    fieldToMethod :: GQL.Field -> f (Maybe (OneMethodQuery p ('Service sname methods)))
-    fieldToMethod f@(GQL.Field alias name args dirs sels _)
-      | any (shouldSkip vmap) dirs
-      = pure Nothing
-      | name == "__typename"
-      = case (args, sels) of
-          ([], []) -> pure $ Just $ TypeNameQuery alias
-          _        -> throwError "__typename does not admit arguments nor selection of subfields"
-      | name == "__schema"
-      = case args of
-          [] -> Just . SchemaQuery alias <$> unFragment frmap (F.toList sels)
-          _  -> throwError "__schema does not admit selection of subfields"
-      | name == "__type"
-      = let getString (GQL.String s)   = Just s
-            getString (GQL.Variable v) = HM.lookup v vmap >>= getString
-            getString _                = Nothing
-        in case args of
-          [GQL.Argument _ (GQL.Node val _) _]
-            -> case getString val of
-                 Just s -> Just . TypeQuery alias s <$> unFragment frmap sels
-                 _      -> throwError "__type requires a string argument"
-          _ -> throwError "__type requires one single argument"
-      | otherwise
-      = Just . OneMethodQuery alias
-         <$> selectMethod (Proxy @('Service s methods))
-                          (T.pack $ nameVal (Proxy @s))
-                          vmap frmap f
-parseQuery pp ps vmap frmap (GQL.FragmentSpreadSelection (GQL.FragmentSpread nm dirs _) : ss)
-  | Just fr <- HM.lookup nm frmap
-  = if not (any (shouldSkip vmap) dirs) && not (any (shouldSkip vmap) $ fdDirectives fr)
-       then (++) <$> parseQuery pp ps vmap frmap (fdSelectionSet fr)
-                 <*> parseQuery pp ps vmap frmap ss
-       else parseQuery pp ps vmap frmap ss
-  | otherwise  -- the fragment definition was not found
-  = throwError $ "fragment '" <> nm <> "' was not found"
-parseQuery _ _ _ _ (_ : _)  -- Inline fragments are not yet supported
-  = throwError "inline fragments are not (yet) supported"
+instance ( p ~ 'Package pname ss
+         , KnownName s
+         , ParseQuery' p s (LookupService ss s) )
+         => ParseQuery p s where
+  parseQuery pp ps = parseQuery' pp ps (Proxy @(LookupService ss s))
+
+class ParseQuery' (p :: Package') (s :: Symbol) (svc :: Service') where
+  parseQuery'
+    :: ( MonadError T.Text f, p ~ 'Package pname ss
+       , LookupService ss s ~ svc, KnownName s )
+    => Proxy p -> Proxy s -> Proxy svc
+    -> VariableMap -> FragmentMap -> [GQL.Selection]
+    -> f (ServiceQuery p svc)
+
+instance (ParseQueryOneOf p elts)
+         => ParseQuery' p s ('OneOf s elts) where
+  parseQuery' pp _ps _ vmap frmap fs
+    = OneOfQuery <$> parseQueryOneOf pp (Proxy @elts) vmap frmap fs
+
+class ParseQueryOneOf (p :: Package') (s :: [Symbol]) where
+  parseQueryOneOf
+    :: ( MonadError T.Text f, p ~ 'Package pname ss )
+    => Proxy p -> Proxy s
+    -> VariableMap -> FragmentMap -> [GQL.Selection]
+    -> f (NP (ChosenOneOfQuery p) s)
+
+instance ParseQueryOneOf p '[] where
+  parseQueryOneOf _ _ _ _ _ = pure Nil
+instance ( ParseQuery p s, KnownSymbol s
+         , ParseQueryOneOf p ss)
+         => ParseQueryOneOf p (s ': ss) where
+  parseQueryOneOf pp _ps vmap frmap sel
+    = (:*) <$> (ChosenOneOfQuery (Proxy @s) <$> parseQuery pp (Proxy @s) vmap frmap sel)
+           <*> parseQueryOneOf pp (Proxy @ss) vmap frmap sel
+
+instance ( ParseMethod p ('Service s methods) methods )
+         => ParseQuery' p s ('Service s methods) where
+  parseQuery' _pp _ps _psvc vmap frmap fs = ServiceQuery <$> go fs
+    where
+      go [] = pure []
+      go (GQL.FieldSelection fld : ss)
+            = (++) <$> (maybeToList <$> fieldToMethod fld) <*> go ss
+      go (GQL.FragmentSpreadSelection (GQL.FragmentSpread nm dirs _) : ss)
+        | Just fr <- HM.lookup nm frmap
+        = if not (any (shouldSkip vmap) dirs) && not (any (shouldSkip vmap) $ fdDirectives fr)
+            then (++) <$> go (fdSelectionSet fr) <*> go ss
+            else go ss
+        | otherwise  -- the fragment definition was not found
+        = throwError $ "fragment '" <> nm <> "' was not found"
+      go (_ : _)  -- Inline fragments are not yet supported
+        = throwError "inline fragments are not (yet) supported"
+      -- fieldToMethod :: GQL.Field -> f (Maybe (OneMethodQuery p ('Service sname methods)))
+      fieldToMethod f@(GQL.Field alias name args dirs sels _)
+        | any (shouldSkip vmap) dirs
+        = pure Nothing
+        | name == "__typename"
+        = case (args, sels) of
+            ([], []) -> pure $ Just $ TypeNameQuery alias
+            _        -> throwError "__typename does not admit arguments nor selection of subfields"
+        | name == "__schema"
+        = case args of
+            [] -> Just . SchemaQuery alias <$> unFragment frmap (F.toList sels)
+            _  -> throwError "__schema does not admit selection of subfields"
+        | name == "__type"
+        = let getString (GQL.String s)   = Just s
+              getString (GQL.Variable v) = HM.lookup v vmap >>= getString
+              getString _                = Nothing
+          in case args of
+            [GQL.Argument _ (GQL.Node val _) _]
+              -> case getString val of
+                  Just s -> Just . TypeQuery alias s <$> unFragment frmap sels
+                  _      -> throwError "__type requires a string argument"
+            _ -> throwError "__type requires one single argument"
+        | otherwise
+        = Just . OneMethodQuery alias
+          <$> selectMethod (Proxy @('Service s methods))
+                            (T.pack $ nameVal (Proxy @s))
+                            vmap frmap f
 
 shouldSkip :: VariableMap -> GQL.Directive -> Bool
 shouldSkip vmap (GQL.Directive nm [GQL.Argument ifn (GQL.Node v _) _] _)
@@ -362,7 +398,7 @@ instance ParseMethod p s '[] where
   selectMethod _ tyName _ _ (fName -> wanted)
     = throwError $ "field '" <> wanted <> "' was not found on type '" <> tyName <> "'"
 instance
-  ( KnownSymbol mname, ParseMethod p s ms
+  ( KnownName mname, ParseMethod p s ms
   , ParseArgs p s ('Method mname args r) args
   , ParseDifferentReturn p r) =>
   ParseMethod p s ('Method mname args r ': ms)
@@ -713,10 +749,8 @@ instance ParseReturn p r
          => ParseReturn p ('OptionalRef r) where
   parseReturn vmap frmap fname s
     = RetOptional <$> parseReturn vmap frmap fname s
-instance ( p ~ 'Package pname ss,
-           LookupService ss s ~ 'Service s methods,
-           KnownName s, ParseMethod p ('Service s methods) methods
-         ) => ParseReturn p ('ObjectRef s) where
+instance ( p ~ 'Package pname ss, ParseQuery p s )
+         => ParseReturn p ('ObjectRef s) where
   parseReturn vmap frmap _ s
     = RetObject <$> parseQuery (Proxy @p) (Proxy @s) vmap frmap s
 
@@ -732,7 +766,7 @@ instance ParseSchema sch ('DEnum name choices) where
     = pure QueryEnum
   parseSchema _ _ fname _
     = throwError $ "field '" <> fname <> "' should not have a selection of subfields"
-instance (KnownSymbol name, ParseField sch fields)
+instance (KnownName name, ParseField sch fields)
          => ParseSchema sch ('DRecord name fields) where
   parseSchema vmap frmap _ s
     = QueryRecord <$> parseSchemaQuery (Proxy @sch) (Proxy @('DRecord name fields)) vmap frmap s
@@ -741,7 +775,7 @@ parseSchemaQuery ::
   forall (sch :: Schema') t (rname :: Symbol) fields f.
   ( MonadError T.Text f
   , t ~  'DRecord rname fields
-  , KnownSymbol rname
+  , KnownName rname
   , ParseField sch fields ) =>
   Proxy sch ->
   Proxy t ->
@@ -790,7 +824,7 @@ instance ParseField sch '[] where
   selectField tyName _ _ wanted _
     = throwError $ "field '" <> wanted <> "' was not found on type '" <> tyName <> "'"
 instance
-  (KnownSymbol fname, ParseField sch fs, ParseSchemaReturn sch r) =>
+  (KnownName fname, ParseField sch fs, ParseSchemaReturn sch r) =>
   ParseField sch ('FieldDef fname r ': fs)
   where
   selectField tyName vmap frmap wanted sels
diff --git a/graphql/src/Mu/GraphQL/Query/Run.hs b/graphql/src/Mu/GraphQL/Query/Run.hs
index ac814d4..b539e3b 100644
--- a/graphql/src/Mu/GraphQL/Query/Run.hs
+++ b/graphql/src/Mu/GraphQL/Query/Run.hs
@@ -11,6 +11,7 @@
 {-# language ScopedTypeVariables   #-}
 {-# language TupleSections         #-}
 {-# language TypeApplications      #-}
+{-# language TypeFamilies          #-}
 {-# language TypeOperators         #-}
 {-# language UndecidableInstances  #-}
 {-# OPTIONS_GHC -fprint-explicit-foralls #-}
@@ -36,9 +37,11 @@ import           Data.Conduit.TQueue
 import qualified Data.HashMap.Strict            as HM
 import           Data.Maybe
 import qualified Data.Text                      as T
+import           Data.Typeable
 import           GHC.TypeLits
 import qualified Language.GraphQL.AST           as GQL
 import           Network.HTTP.Types.Header
+import           Unsafe.Coerce                  (unsafeCoerce)
 
 import           Mu.GraphQL.Query.Definition
 import qualified Mu.GraphQL.Query.Introspection as Intro
@@ -233,11 +236,10 @@ yieldDocument f req svr doc sink = do
   runConduit $ yieldMany ([val] :: [Aeson.Value]) .| sink
 
 runQuery
-  :: forall m p s pname ss hs sname ms chn inh.
+  :: forall m p s pname ss hs chn inh.
      ( RunQueryFindHandler m p hs chn ss s hs
      , p ~ 'Package pname ss
-     , s ~ 'Service sname ms
-     , inh ~ MappingRight chn sname )
+     , inh ~ MappingRight chn (ServiceName s) )
   => (forall a. m a -> ServerErrorIO a)
   -> RequestHeaders
   -> Intro.Schema -> ServerT chn GQL.Field p m hs
@@ -248,11 +250,10 @@ runQuery
 runQuery f req sch whole@(Services ss) path = runQueryFindHandler f req sch whole path ss
 
 runSubscription
-  :: forall m p s pname ss hs sname ms chn inh.
+  :: forall m p s pname ss hs chn inh.
      ( RunQueryFindHandler m p hs chn ss s hs
      , p ~ 'Package pname ss
-     , s ~ 'Service sname ms
-     , inh ~ MappingRight chn sname )
+     , inh ~ MappingRight chn (ServiceName s) )
   => (forall a. m a -> ServerErrorIO a)
   -> RequestHeaders
   -> ServerT chn GQL.Field p m hs
@@ -267,8 +268,7 @@ runSubscription f req whole@(Services ss) path
 class RunQueryFindHandler m p whole chn ss s hs where
   runQueryFindHandler
     :: ( p ~ 'Package pname wholess
-       , s ~ 'Service sname ms
-       , inh ~ MappingRight chn sname )
+       , inh ~ MappingRight chn (ServiceName s) )
     => (forall a. m a -> ServerErrorIO a)
     -> RequestHeaders
     -> Intro.Schema -> ServerT chn GQL.Field p m whole
@@ -279,8 +279,7 @@ class RunQueryFindHandler m p whole chn ss s hs where
     -> WriterT [GraphQLError] IO Aeson.Value
   runSubscriptionFindHandler
     :: ( p ~ 'Package pname wholess
-       , s ~ 'Service sname ms
-       , inh ~ MappingRight chn sname )
+       , inh ~ MappingRight chn (ServiceName s) )
     => (forall a. m a -> ServerErrorIO a)
     -> RequestHeaders
     -> ServerT chn GQL.Field p m whole
@@ -291,6 +290,32 @@ class RunQueryFindHandler m p whole chn ss s hs where
     -> ConduitT Aeson.Value Void IO ()
     -> IO ()
 
+class RunQueryOnFoundHandler m p whole chn (s :: Service snm mnm anm (TypeRef snm)) hs where
+  type ServiceName s :: snm
+  runQueryOnFoundHandler
+    :: ( p ~ 'Package pname wholess
+       , inh ~ MappingRight chn (ServiceName s) )
+    => (forall a. m a -> ServerErrorIO a)
+    -> RequestHeaders
+    -> Intro.Schema -> ServerT chn GQL.Field p m whole
+    -> [T.Text]
+    -> ServiceT chn GQL.Field s m hs
+    -> inh
+    -> ServiceQuery p s
+    -> WriterT [GraphQLError] IO Aeson.Value
+  runSubscriptionOnFoundHandler
+    :: ( p ~ 'Package pname wholess
+       , inh ~ MappingRight chn (ServiceName s) )
+    => (forall a. m a -> ServerErrorIO a)
+    -> RequestHeaders
+    -> ServerT chn GQL.Field p m whole
+    -> [T.Text]
+    -> ServiceT chn GQL.Field s m hs
+    -> inh
+    -> OneMethodQuery p s
+    -> ConduitT Aeson.Value Void IO ()
+    -> IO ()
+
 instance TypeError ('Text "Could not find handler for " ':<>: 'ShowType s)
          => RunQueryFindHandler m p whole chn '[] s '[] where
   runQueryFindHandler _ = error "this should never be called"
@@ -302,17 +327,24 @@ instance {-# OVERLAPPABLE #-}
     = runQueryFindHandler f req sch whole path that
   runSubscriptionFindHandler f req whole path (_ :<&>: that)
     = runSubscriptionFindHandler f req whole path that
-instance {-# OVERLAPS #-}
-         ( s ~ 'Service sname ms, KnownName sname
-         , RunMethod m p whole chn s ms h )
+instance {-# OVERLAPS #-}
+         (RunQueryOnFoundHandler m p whole chn s h)
          => RunQueryFindHandler m p whole chn (s ': ss) s (h ': hs) where
-  runQueryFindHandler f req sch whole path (this :<&>: _) inh queries
+  runQueryFindHandler f req sch whole path (s :<&>: _)
+    = runQueryOnFoundHandler f req sch whole path s
+  runSubscriptionFindHandler f req whole path (s :<&>: _)
+    = runSubscriptionOnFoundHandler f req whole path s
+
+instance ( KnownName sname, RunMethod m p whole chn ('Service sname ms) ms h )
+         => RunQueryOnFoundHandler m p whole chn ('Service sname ms) h where
+  type ServiceName ('Service sname ms) = sname
+  runQueryOnFoundHandler f req sch whole path (ProperSvc this) inh (ServiceQuery queries)
     = Aeson.object . catMaybes <$> mapM runOneQuery queries
     where
       -- if we include the signature we have to write
       -- an explicit type signature for 'runQueryFindHandler'
       runOneQuery (OneMethodQuery nm args)
-        = runMethod f req whole (Proxy @s) path nm inh this args
+        = runMethod f req whole (Proxy @('Service sname ms)) path nm inh this args
       -- handle __typename
       runOneQuery (TypeNameQuery nm)
         = let realName = fromMaybe "__typename" nm
@@ -333,23 +365,59 @@ instance {-# OVERLAPS #-}
                                     path]
                               pure $ Just (realName, Aeson.Null)
   -- subscriptions should only have one element
-  runSubscriptionFindHandler f req whole path (this :<&>: _) inh (OneMethodQuery nm args) sink
-    = runMethodSubscription f req whole (Proxy @s) path nm inh this args sink
-  runSubscriptionFindHandler _ _ _ _ _ _ (TypeNameQuery nm) sink
+  runSubscriptionOnFoundHandler f req whole path (ProperSvc this) inh (OneMethodQuery nm args) sink
+    = runMethodSubscription f req whole (Proxy @('Service sname ms)) path nm inh this args sink
+  runSubscriptionOnFoundHandler _ _ _ _ _ _ (TypeNameQuery nm) sink
     = let realName = fromMaybe "__typename" nm
           o = Aeson.object [(realName, Aeson.String $ T.pack $ nameVal (Proxy @sname))]
       in runConduit $ yieldMany ([o] :: [Aeson.Value]) .| sink
-  runSubscriptionFindHandler _ _ _ _ _ _ _ sink
+  runSubscriptionOnFoundHandler _ _ _ _ _ _ _ sink
     = runConduit $ yieldMany
                    ([singleErrValue "__schema and __type are not supported in subscriptions"]
                       :: [Aeson.Value])
                    .| sink
 
+instance ( KnownName sname, RunUnion m p whole chn elts )
+         => RunQueryOnFoundHandler m p whole chn ('OneOf sname elts) h where
+  type ServiceName ('OneOf sname elts) = sname
+  runQueryOnFoundHandler f req sch whole path (OneOfSvc this) inh (OneOfQuery queries)
+    = do res <- liftIO $ runExceptT $ f $ this inh
+         case res of
+          Left e  -> tell [GraphQLError e path] >> pure Aeson.Null
+          Right x -> runUnion f req sch whole path queries x
+  runSubscriptionOnFoundHandler _ _ _ _ (OneOfSvc _) _ _ _
+    = error "this should never happen"
+
+class RunUnion m p whole chn elts where
+  runUnion
+    :: (forall a. m a -> ServerErrorIO a)
+    -> RequestHeaders
+    -> Intro.Schema -> ServerT chn GQL.Field p m whole
+    -> [T.Text]
+    -> NP (ChosenOneOfQuery p) elts
+    -> UnionChoice chn elts
+    -> WriterT [GraphQLError] IO Aeson.Value
+
+instance RunUnion m p whole chn '[] where
+  runUnion _ = error "this should never happen"
+instance forall m p pname s sname whole ss chn elts.
+         ( RunQueryFindHandler m p whole chn ss s whole
+         , p ~ 'Package pname ss
+         , s ~ LookupService ss sname
+         , ServiceName s ~ sname
+         , RunUnion m p whole chn elts )
+         => RunUnion m p whole chn (sname ': elts) where
+  runUnion f req sch whole path
+           (ChosenOneOfQuery (Proxy :: Proxy sname) q :* rest)
+           choice@(UnionChoice (Proxy :: Proxy other) v)
+    = case eqT @sname @other of
+        Nothing   -> runUnion f req sch whole path rest (unsafeCoerce choice)
+        Just Refl -> runQuery @m @('Package pname ss) @(LookupService ss sname) @pname @ss @whole f req sch whole path v q
+
 class RunMethod m p whole chn s ms hs where
   runMethod
     :: ( p ~ 'Package pname wholess
-       , s ~ 'Service sname allMs
-       , inh ~ MappingRight chn sname )
+       , inh ~ MappingRight chn (ServiceName s) )
     => (forall a. m a -> ServerErrorIO a)
     -> RequestHeaders
     -> ServerT chn GQL.Field p m whole
@@ -359,8 +427,7 @@ class RunMethod m p whole chn s ms hs where
     -> WriterT [GraphQLError] IO (Maybe (T.Text, Aeson.Value))
   runMethodSubscription
     :: ( p ~ 'Package pname wholess
-       , s ~ 'Service sname allMs
-       , inh ~ MappingRight chn sname )
+       , inh ~ MappingRight chn (ServiceName s) )
     => (forall a. m a -> ServerErrorIO a)
     -> RequestHeaders
     -> ServerT chn GQL.Field p m whole
@@ -385,6 +452,7 @@ instance ( RunMethod m p whole chn s ms hs
           rpcInfo = reflectRpcInfo (Proxy @p) (Proxy @s) (Proxy @('Method mname args r)) req fld
   runMethod f req whole p path nm inh (_ :<||>: r) (S cont)
     = runMethod f req whole p path nm inh r cont
+  runMethod _ _ _ _ _ _ _ _ _ = error "this should never happen"
   -- handle subscriptions
   runMethodSubscription f req whole _ path nm inh (h :<||>: _) (Z (ChosenMethodQuery fld args ret)) sink
     = runHandlerSubscription f req whole (path ++ [realName]) (h rpcInfo inh) args ret sink
@@ -392,6 +460,7 @@ instance ( RunMethod m p whole chn s ms hs
           rpcInfo = reflectRpcInfo (Proxy @p) (Proxy @s) (Proxy @('Method mname args r)) req fld
   runMethodSubscription f req whole p path nm inh (_ :<||>: r) (S cont) sink
     = runMethodSubscription f req whole p path nm inh r cont sink
+  runMethodSubscription _ _ _ _ _ _ _ _ _ _ = error "this should never happen"
 
 class Handles chn args r m h
       => RunHandler m p whole chn args r h where
@@ -518,9 +587,9 @@ instance ( ToSchema sch l r
   convertResult _ _ _ _ (RetSchema r) t
     = pure $ Just $ runSchemaQuery (toSchema' @_ @_ @sch @r t) r
 instance ( MappingRight chn ref ~ t
-         , MappingRight chn sname ~ t
-         , LookupService ss ref ~ 'Service sname ms
-         , RunQueryFindHandler m ('Package pname ss) whole chn ss ('Service sname ms) whole)
+         , MappingRight chn (ServiceName svc) ~ t
+         , LookupService ss ref ~ svc
+         , RunQueryFindHandler m ('Package pname ss) whole chn ss svc whole)
          => ResultConversion m ('Package pname ss) whole chn ('ObjectRef ref) t where
   convertResult f req whole path (RetObject q) h
     = Just <$> runQuery @m @('Package pname ss) @(LookupService ss ref) f req
@@ -643,7 +712,7 @@ runIntroType path s@(Intro.Schema _ _ _ ts) (Intro.TypeRef t) ss
   = case HM.lookup t ts of
       Nothing -> pure Nothing
       Just ty -> runIntroType path s ty ss
-runIntroType path s (Intro.Type k tnm fs vals ofT) ss
+runIntroType path s (Intro.Type k tnm fs vals posTys ofT) ss
   = do things <- catMaybes <$> traverse runOne ss
        pure $ Just $ Aeson.object things
   where
@@ -683,7 +752,12 @@ runIntroType path s (Intro.Type k tnm fs vals ofT) ss
              ("interfaces", _)
                -> pure $ Just $ Aeson.Array []
              ("possibleTypes", _)
-               -> pure $ Just $ Aeson.Array []
+               -> case k of
+                    Intro.UNION
+                      -> do res <- catMaybes <$>
+                                     mapM (\o -> runIntroType path' s o innerss) posTys
+                            pure $ Just $ Aeson.toJSON res
+                    _ -> pure $ Just Aeson.Null
 
              _ -> do tell [GraphQLError
                              (ServerError Invalid
diff --git a/grpc/client/mu-grpc-client.cabal b/grpc/client/mu-grpc-client.cabal
index bf46ddb..1b98906 100644
--- a/grpc/client/mu-grpc-client.cabal
+++ b/grpc/client/mu-grpc-client.cabal
@@ -41,7 +41,7 @@ library
     , mu-grpc-common     >=0.4   && <0.5
     , mu-optics          >=0.3   && <0.4
     , mu-protobuf        >=0.4   && <0.5
-    , mu-rpc             >=0.4   && <0.5
+    , mu-rpc             >=0.4   && <0.6
     , mu-schema          >=0.3   && <0.4
     , optics-core        >=0.2   && <0.4
     , sop-core           >=0.5   && <0.6
diff --git a/grpc/common/mu-grpc-common.cabal b/grpc/common/mu-grpc-common.cabal
index 3a8d08e..0a9588c 100644
--- a/grpc/common/mu-grpc-common.cabal
+++ b/grpc/common/mu-grpc-common.cabal
@@ -33,7 +33,7 @@ library
     , http2-grpc-types        >=0.5   && <0.6
     , mu-avro                 >=0.4   && <0.5
     , mu-protobuf             >=0.4   && <0.5
-    , mu-rpc                  >=0.4   && <0.5
+    , mu-rpc                  >=0.4   && <0.6
     , mu-schema               >=0.3   && <0.4
 
   hs-source-dirs:   src
diff --git a/grpc/server/mu-grpc-server.cabal b/grpc/server/mu-grpc-server.cabal
index c37fde2..3ca0c0f 100644
--- a/grpc/server/mu-grpc-server.cabal
+++ b/grpc/server/mu-grpc-server.cabal
@@ -1,5 +1,5 @@
 name:               mu-grpc-server
-version:            0.4.0.0
+version:            0.5.0.0
 synopsis:           gRPC servers for Mu definitions
 description:
   With @mu-grpc-server@ you can easily build gRPC servers for mu-haskell!
@@ -33,7 +33,7 @@ library
     , mtl               >=2.2     && <3
     , mu-grpc-common    >=0.4     && <0.5
     , mu-protobuf       >=0.4     && <0.5
-    , mu-rpc            >=0.4     && <0.5
+    , mu-rpc            >=0.5     && <0.6
     , mu-schema         >=0.3     && <0.4
     , sop-core          >=0.5     && <0.6
     , stm               >=2.5     && <3
@@ -59,9 +59,9 @@ executable grpc-example-server
     , http2-grpc-types  >=0.5     && <0.6
     , mtl               >=2.2     && <3
     , mu-grpc-common    >=0.4     && <0.5
-    , mu-grpc-server    >=0.4     && <0.5
+    , mu-grpc-server
     , mu-protobuf       >=0.4     && <0.5
-    , mu-rpc            >=0.4     && <0.5
+    , mu-rpc            >=0.5     && <0.6
     , mu-schema         >=0.3     && <0.4
     , sop-core          >=0.5     && <0.6
     , stm               >=2.5     && <3
diff --git a/grpc/server/src/Mu/GRpc/Server.hs b/grpc/server/src/Mu/GRpc/Server.hs
index 06d9fde..8041461 100644
--- a/grpc/server/src/Mu/GRpc/Server.hs
+++ b/grpc/server/src/Mu/GRpc/Server.hs
@@ -47,6 +47,7 @@ import           Data.Conduit
 import           Data.Conduit.TMChan
 import           Data.Kind
 import           Data.Proxy
+import           GHC.TypeLits
 import           Network.GRPC.HTTP2.Encoding        (GRPCInput, GRPCOutput, gzip, uncompressed)
 import           Network.GRPC.HTTP2.Types           (GRPCStatus (..), GRPCStatusCode (..))
 import           Network.GRPC.Server.Handlers.Trans
@@ -174,12 +175,15 @@ instance ( KnownName name
                               p m chn (MappingRight chn name) methods h
          , GRpcServiceHandlers fullP p m chn rest hs )
          => GRpcServiceHandlers fullP p m chn ('Service name methods ': rest) (h ': hs) where
-  gRpcServiceHandlers f pfullP pr packageName (svr :<&>: rest)
+  gRpcServiceHandlers f pfullP pr packageName (ProperSvc svr :<&>: rest)
     =  gRpcMethodHandlers f pfullP (Proxy @('Service name methods)) pr
                           packageName serviceName svr
     ++ gRpcServiceHandlers f pfullP pr packageName rest
     where serviceName = BS.pack (nameVal (Proxy @name))
 
+instance ( GHC.TypeLits.TypeError ('Text "unions are not supported in gRPC") )
+         => GRpcServiceHandlers fullP p m chn ('OneOf name methods ': rest) hs where
+  gRpcServiceHandlers _ = error "unions are not supported in gRPC"
 
 class GRpcMethodHandlers (fullP :: Package snm mnm anm (TypeRef snm))
                          (fullS :: Service snm mnm anm (TypeRef snm))
diff --git a/instrumentation/prometheus/mu-prometheus.cabal b/instrumentation/prometheus/mu-prometheus.cabal
index b83e936..97d5c0a 100644
--- a/instrumentation/prometheus/mu-prometheus.cabal
+++ b/instrumentation/prometheus/mu-prometheus.cabal
@@ -1,5 +1,5 @@
 name:          mu-prometheus
-version:       0.4.0.0
+version:       0.5.0.0
 synopsis:      Metrics support for Mu using Prometheus
 description:   Get metrics of your running Mu servers using Prometheus
 license:       Apache-2.0
@@ -22,12 +22,12 @@ library
   build-depends:
       base                       >=4.12 && <5
     , lifted-base                >=0.2  && <0.3
-    , monad-control              >1    && <2
-    , mu-rpc                     >=0.4  && <0.5
-    , prometheus-client          >1    && <2
+    , monad-control              >1     && <2
+    , mu-rpc                     >=0.5  && <0.6
+    , prometheus-client          >1     && <2
     , text                       >=1.2  && <2
     , wai                        >=3.2  && <4
-    , wai-middleware-prometheus  >1    && <2
+    , wai-middleware-prometheus  >1     && <2
 
   hs-source-dirs:   src
   default-language: Haskell2010
diff --git a/instrumentation/prometheus/src/Mu/Instrumentation/Prometheus.hs b/instrumentation/prometheus/src/Mu/Instrumentation/Prometheus.hs
index 28e7c99..59c10ce 100644
--- a/instrumentation/prometheus/src/Mu/Instrumentation/Prometheus.hs
+++ b/instrumentation/prometheus/src/Mu/Instrumentation/Prometheus.hs
@@ -49,17 +49,23 @@ prometheusMetrics :: forall m a info. (MonadBaseControl IO m, MonadMonitor m)
 prometheusMetrics metrics NoRpcInfo run = do
   incGauge (activeCalls metrics)
   run `finally` decGauge (activeCalls metrics)
-prometheusMetrics metrics (RpcInfo _pkg (Service sname _) (Method mname _ _) _ _) run = do
+prometheusMetrics metrics (RpcInfo _pkg ss mm _ _) run = do
+  let sname' = case ss of
+                 Service sname _ -> sname
+                 OneOf   sname _ -> sname
+      mname' = case mm of
+                 Just (Method mname _ _) -> mname
+                 Nothing                 -> "<noname>"
   incGauge (activeCalls metrics)
-  withLabel (messagesReceived metrics) (sname, mname) incCounter
+  withLabel (messagesReceived metrics) (sname', mname') incCounter
   ( do -- We are forced to use a MVar because 'withLabel' only allows IO ()
        r <- liftBaseWith $ \runInIO -> do
          result :: MVar (StM m a) <- newEmptyMVar
-         withLabel (callsTotal metrics) (sname, mname) $ \h ->
+         withLabel (callsTotal metrics) (sname', mname') $ \h ->
            h `observeDuration` (runInIO run >>= putMVar result)
          takeMVar result
        x <- restoreM r
-       withLabel (messagesSent metrics) (sname, mname) incCounter
+       withLabel (messagesSent metrics) (sname', mname') incCounter
        pure x )
   `finally` decGauge (activeCalls metrics)
 
diff --git a/servant/server/mu-servant-server.cabal b/servant/server/mu-servant-server.cabal
index 5a45334..1fda79e 100644
--- a/servant/server/mu-servant-server.cabal
+++ b/servant/server/mu-servant-server.cabal
@@ -1,5 +1,5 @@
 name:               mu-servant-server
-version:            0.4.0.0
+version:            0.5.0.0
 synopsis:           Servant servers for Mu definitions
 description:
   With @mu-servant-server@ you can easily build Servant servers for mu-haskell!
@@ -30,7 +30,7 @@ library
     , generic-aeson    >=0.2   && <0.3
     , ghc-prim         >=0.5   && <0.7
     , mtl              >=2.2   && <3
-    , mu-rpc           >=0.4   && <0.5
+    , mu-rpc           >=0.5   && <0.6
     , mu-schema        >=0.3   && <0.4
     , servant          >=0.16  && <0.19
     , servant-server   >=0.16  && <0.19
@@ -49,7 +49,7 @@ executable servant-example-server
       aeson              >=1.4   && <2
     , base               >=4.12  && <5
     , conduit            >=1.3.2 && <2
-    , mu-rpc             >=0.4   && <0.5
+    , mu-rpc             >=0.5   && <0.6
     , mu-schema          >=0.3   && <0.4
     , mu-servant-server
     , servant-server     >=0.16  && <0.19
diff --git a/servant/server/src/Mu/Servant/Server.hs b/servant/server/src/Mu/Servant/Server.hs
index 345339f..fa1a8a5 100644
--- a/servant/server/src/Mu/Servant/Server.hs
+++ b/servant/server/src/Mu/Servant/Server.hs
@@ -47,10 +47,10 @@ import qualified Data.ByteString.Lazy.UTF8 as LB8
 import           Data.Conduit.Internal     (ConduitT (..), Pipe (..))
 import           Data.Kind
 import           Data.Swagger              (Swagger, ToSchema (..))
-import           Generics.Generic.Aeson
 import           GHC.Generics
 import           GHC.TypeLits
 import           GHC.Types                 (Any)
+import           Generics.Generic.Aeson
 import           Mu.Rpc
 import           Mu.Rpc.Annotations
 import           Mu.Schema
@@ -170,10 +170,16 @@ instance
   type
     ServicesAPI pkg ('Service sname methods ': rest) (hs ': hss) =
       MethodsAPI pkg sname methods hs :<|> ServicesAPI pkg rest hss
-  servantServiceHandlers f pkgP (svr :<&>: rest) =
+  servantServiceHandlers f pkgP (ProperSvc svr :<&>: rest) =
     servantMethodHandlers f pkgP (Proxy @sname) svr
       :<|> servantServiceHandlers f pkgP rest
 
+instance (TypeError ('Text "unions are not supported by Servant servers"))
+         => ServantServiceHandlers pkg m chn ('OneOf sname methods ': rest) hs where
+  type ServicesAPI pkg ('OneOf sname methods ': rest) hs =
+    TypeError ('Text "unions are not supported by Servant servers")
+  servantServiceHandlers _ = error "unions are not supported by Servant servers"
+
 class
   ServantMethodHandlers
     (pkg :: Package Symbol Symbol anm (TypeRef Symbol))