diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,11 @@
+1.3.7
+
+* Add `groupBy`
+
+1.3.6
+
+* Documentation improvements
+
 1.3.5
 
 * Add `Choice` instance for `Fold`
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# `foldl` v1.3.5
+# `foldl` v1.3.7
 
 Use this `foldl` library when you want to compute multiple folds over a
 collection in one pass over the data without space leaks.
diff --git a/foldl.cabal b/foldl.cabal
--- a/foldl.cabal
+++ b/foldl.cabal
@@ -1,5 +1,5 @@
 Name: foldl
-Version: 1.3.5
+Version: 1.3.7
 Cabal-Version: >=1.8.0.2
 Build-Type: Simple
 License: BSD3
diff --git a/src/Control/Foldl.hs b/src/Control/Foldl.hs
--- a/src/Control/Foldl.hs
+++ b/src/Control/Foldl.hs
@@ -35,6 +35,7 @@
     executables that use this library to further improve performance.
 -}
 
+{-# LANGUAGE BangPatterns              #-}
 {-# LANGUAGE CPP                       #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE FlexibleContexts          #-}
@@ -130,6 +131,7 @@
     , foldOverM
     , folded
     , filtered
+    , groupBy
 
     -- * Re-exports
     -- $reexports
@@ -146,6 +148,8 @@
 import Data.Foldable (Foldable)
 import Data.Functor.Identity (Identity, runIdentity)
 import Data.Functor.Contravariant (Contravariant(..))
+import Data.Map.Strict (Map, alter)
+import Data.Maybe (fromMaybe)
 import Data.Monoid hiding ((<>))
 import Data.Semigroup (Semigroup(..))
 import Data.Profunctor
@@ -1134,6 +1138,14 @@
 
   This can also be done with 'handles' (@handles (filtered f)@) but @prefilter@
   does not need you to depend on a lens library.
+
+> fold (prefilter p folder) list = fold folder (filter p list)
+
+>>> fold (prefilter (>5) Control.Foldl.sum) [1..10]
+40
+
+>>> fold Control.Foldl.sum (filter (>5) [1..10])
+40
 -}
 prefilter :: (a -> Bool) -> Fold a r -> Fold a r
 prefilter f (Fold step begin done) = Fold step' begin done
@@ -1143,6 +1155,8 @@
 
 {-| @(prefilterM f folder)@ returns a new 'Fold' where the folder's input is used
   only when the input satisfies a monadic predicate f.
+
+> foldM (prefilterM p folder) list = foldM folder (filter p list)
 -}
 prefilterM :: (Monad m) => (a -> m Bool) -> FoldM m a r -> FoldM m a r
 prefilterM f (FoldM step begin done) = FoldM step' begin done
@@ -1304,6 +1318,17 @@
     | p x = k x
     | otherwise = mempty
 {-# INLINABLE filtered #-}
+
+{-| Perform a 'Fold' while grouping the data according to a specified group
+projection function. Returns the folded result grouped as a map keyed by the
+group.
+
+-}
+groupBy :: Ord g => (a -> g) -> Fold a r -> Fold a (Map g r)
+groupBy grouper (Fold f i e) = Fold f' mempty (fmap e)
+  where
+    f' !m !a = alter (\o -> Just (f (fromMaybe i o) a)) (grouper a) m
+{-# INLINABLE groupBy #-}
 
 {- $reexports
     @Control.Monad.Primitive@ re-exports the 'PrimMonad' type class