diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Revision history for ecstasy
 
+## 0.1.1.0  -- 2018-02-18
+
+* Added 'deleteEntity' (function) and 'delEntity' (QueryT setter).
+
 ## 0.1.0.1  -- 2018-02-14
 
 * Added 'yieldSystemT' for resuming a 'SystemT' computation later.
diff --git a/ecstasy.cabal b/ecstasy.cabal
--- a/ecstasy.cabal
+++ b/ecstasy.cabal
@@ -2,7 +2,7 @@
 -- documentation, see http://haskell.org/cabal/users-guide/
 
 name:                ecstasy
-version:             0.1.0.1
+version:             0.1.1.0
 synopsis:
   A GHC.Generics based entity component system.
 
diff --git a/src/Data/Ecstasy.hs b/src/Data/Ecstasy.hs
--- a/src/Data/Ecstasy.hs
+++ b/src/Data/Ecstasy.hs
@@ -3,6 +3,7 @@
 {-# LANGUAGE FlexibleContexts     #-}
 {-# LANGUAGE FlexibleInstances    #-}
 {-# LANGUAGE TupleSections        #-}
+{-# LANGUAGE TypeApplications     #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE ViewPatterns         #-}
 
@@ -98,30 +99,40 @@
   defEntity :: world 'FieldOf
   default defEntity
       :: ( Generic (world 'FieldOf)
-         , GDefault (Rep (world 'FieldOf))
+         , GDefault 'True (Rep (world 'FieldOf))
          )
       => world 'FieldOf
-  defEntity = def
+  defEntity = def @'True
 
   ----------------------------------------------------------------------------
   -- | The default setter, which keeps all components with their previous value.
   defEntity' :: world 'SetterOf
   default defEntity'
       :: ( Generic (world 'SetterOf)
-         , GDefault (Rep (world 'SetterOf))
+         , GDefault 'True (Rep (world 'SetterOf))
          )
       => world 'SetterOf
-  defEntity' = def
+  defEntity' = def @'True
 
   ----------------------------------------------------------------------------
+  -- | A setter which will delete the entity if its 'QueryT' matches.
+  delEntity :: world 'SetterOf
+  default delEntity
+      :: ( Generic (world 'SetterOf)
+         , GDefault 'False (Rep (world 'SetterOf))
+         )
+      => world 'SetterOf
+  delEntity = def @'False
+
+  ----------------------------------------------------------------------------
   -- | The default world, which contains only empty containers.
   defWorld :: world 'WorldOf
   default defWorld
       :: ( Generic (world 'WorldOf)
-         , GDefault (Rep (world 'WorldOf))
+         , GDefault 'True (Rep (world 'WorldOf))
          )
       => world 'WorldOf
-  defWorld = def
+  defWorld = def @'True
 
 
 instance ( Generic (world 'SetterOf)
@@ -133,9 +144,10 @@
                       (Rep (world 'FieldOf))
          , GConvertSetter (Rep (world 'FieldOf))
                           (Rep (world 'SetterOf))
-         , GDefault (Rep (world 'FieldOf))
-         , GDefault (Rep (world 'SetterOf))
-         , GDefault (Rep (world 'WorldOf))
+         , GDefault 'True  (Rep (world 'FieldOf))
+         , GDefault 'False (Rep (world 'SetterOf))
+         , GDefault 'True  (Rep (world 'SetterOf))
+         , GDefault 'True  (Rep (world 'WorldOf))
          ) => HasWorld world
 
 
@@ -160,6 +172,15 @@
   e <- nextEntity
   setEntity e $ convertSetter cs
   pure e
+
+
+------------------------------------------------------------------------------
+-- | Delete an entity.
+deleteEntity
+    :: (HasWorld world, Monad m)
+    => Ent
+    -> SystemT world m ()
+deleteEntity = flip setEntity delEntity
 
 
 ------------------------------------------------------------------------------
diff --git a/src/Data/Ecstasy/Deriving.hs b/src/Data/Ecstasy/Deriving.hs
--- a/src/Data/Ecstasy/Deriving.hs
+++ b/src/Data/Ecstasy/Deriving.hs
@@ -1,6 +1,11 @@
+{-# LANGUAGE AllowAmbiguousTypes   #-}
+{-# LANGUAGE DataKinds             #-}
 {-# LANGUAGE FlexibleContexts      #-}
 {-# LANGUAGE FlexibleInstances     #-}
+{-# LANGUAGE KindSignatures        #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables   #-}
+{-# LANGUAGE TypeApplications      #-}
 {-# LANGUAGE TypeOperators         #-}
 
 module Data.Ecstasy.Deriving where
@@ -84,35 +89,39 @@
   {-# INLINE gSetEntity #-}
 
 
-def :: (Generic a, GDefault (Rep a)) => a
-def = to gdef
+def :: forall keep a. (Generic a, GDefault keep (Rep a)) => a
+def = to $ gdef @keep
 {-# INLINE def #-}
 
 
-class GDefault f where
+class GDefault (keep :: Bool) f where
   gdef :: f a
 
-instance GDefault U1 where
+instance GDefault keep U1 where
   gdef = U1
   {-# INLINE gdef #-}
 
-instance GDefault (K1 i (Maybe c)) where
+instance GDefault keep (K1 i (Maybe c)) where
   gdef = K1 Nothing
   {-# INLINE gdef #-}
 
-instance GDefault (K1 i (Update c)) where
+instance GDefault 'False (K1 i (Update c)) where
+  gdef = K1 Unset
+  {-# INLINE gdef #-}
+
+instance GDefault 'True (K1 i (Update c)) where
   gdef = K1 Keep
   {-# INLINE gdef #-}
 
-instance GDefault (K1 i (IntMap c)) where
+instance GDefault keep (K1 i (IntMap c)) where
   gdef = K1 I.empty
   {-# INLINE gdef #-}
 
-instance GDefault f => GDefault (M1 i c f) where
-  gdef = M1 gdef
+instance GDefault keep f => GDefault keep (M1 i c f) where
+  gdef = M1 $ gdef @keep
   {-# INLINE gdef #-}
 
-instance (GDefault a, GDefault b) => GDefault (a :*: b) where
-  gdef = gdef :*: gdef
+instance (GDefault keep a, GDefault keep b) => GDefault keep (a :*: b) where
+  gdef = gdef @keep :*: gdef @keep
   {-# INLINE gdef #-}