diff --git a/Command/Fsck.hs b/Command/Fsck.hs
index bd5a9ad7f0..c86f30ff80 100644
--- a/Command/Fsck.hs
+++ b/Command/Fsck.hs
@@ -7,15 +7,10 @@
 
 module Command.Fsck where
 
-import Control.Monad.State (liftIO)
-import System.Posix.Files
-import System.Directory
-
 import Command
-import qualified Annex
 import Types
-import Utility
 import Core
+import qualified Data.Map as M
 
 {- Checks the whole annex for problems. -}
 start :: SubCmdStart
@@ -35,5 +30,40 @@ perform = do
 checkUnused :: Annex Bool
 checkUnused = do
 	showNote "checking for unused data..."
-	-- TODO
-	return False
+	unused <- unusedKeys
+	if (null unused)
+		then return True
+		else do
+			showLongNote $ w unused
+			return False
+	where
+		w u = unlines $ [
+			"Some annexed data is no longer pointed to by any file.",
+			"If this data is no longer needed, it can be removed using git-annex dropkey:"
+			] ++ map show u
+
+unusedKeys :: Annex [Key]
+unusedKeys = do
+	present <- getKeysPresent
+	referenced <- getKeysReferenced
+	
+	-- Constructing a single map, of the set that tends to be smaller,
+	-- appears more efficient in both memory and CPU than constructing
+	-- and taking the M.difference of two maps.
+	let present_m = existsMap present
+	let unused_m = remove referenced present_m
+	return $ M.keys unused_m
+	where
+		remove [] m = m
+		remove (x:xs) m = remove xs $ M.delete x m
+
+existsMap :: Ord k => [k] -> M.Map k Int
+existsMap l = M.fromList $ map (\k -> (k, 1)) l
+
+getKeysPresent :: Annex [Key]
+getKeysPresent = do
+	return []
+
+getKeysReferenced :: Annex [Key]
+getKeysReferenced = do
+	return []
diff --git a/TypeInternals.hs b/TypeInternals.hs
index 46c92cb59b..4b5cff9d9f 100644
--- a/TypeInternals.hs
+++ b/TypeInternals.hs
@@ -39,7 +39,7 @@ type Annex = StateT AnnexState IO
 -- annexed filenames are mapped through a backend into keys
 type KeyName = String
 type BackendName = String
-data Key = Key (BackendName, KeyName) deriving (Eq)
+data Key = Key (BackendName, KeyName) deriving (Eq, Ord)
 
 -- constructs a key in a backend
 genKey :: Backend -> KeyName -> Key