diff --git a/Data/Git/Commit.hs b/Data/Git/Commit.hs
--- a/Data/Git/Commit.hs
+++ b/Data/Git/Commit.hs
@@ -99,17 +99,6 @@
                       , _commitEncoding  = encs
                       , _commitObj       = Just $ unsafeCoerce obj }
 
-withGitCommit :: Updatable b
-              => ObjRef Commit -> b -> (Ptr C'git_commit -> IO a) -> IO a
-withGitCommit cref obj f = do
-  c' <- loadObject cref obj
-  case c' of
-    Nothing -> error "Cannot find Git commit in repository"
-    Just co ->
-      case co^.commitInfo.gitObj of
-        Nothing  -> error "Cannot find Git commit id"
-        Just co' -> withForeignPtr co' (f . castPtr)
-
 -- | Write out a commit to its repository.  If it has already been written,
 --   nothing will happen.
 writeCommit :: Maybe Text -> Commit -> IO Commit
@@ -182,6 +171,22 @@
               when (r < 0) $ throwIO CommitLookupFailed
               ptr' <- peek ptr
               FC.newForeignPtr ptr' (c'git_commit_free ptr')
+
+modifyCommitTree
+  :: FilePath -> (Maybe TreeEntry -> Either a (Maybe TreeEntry)) -> Bool
+  -> Commit -> IO (Either a Commit)
+modifyCommitTree path f createIfNotExist c =
+  withObject (c^.commitTree) c $ \tr -> do
+    result <- modifyTree path f createIfNotExist tr
+    case result of
+      Left x    -> return (Left x)
+      Right tr' -> return $ Right $ commitTree .~ ObjRef tr' $ c
+
+removeFromCommitTree :: FilePath -> Commit -> IO Commit
+removeFromCommitTree path c =
+  withObject (c^.commitTree) c $ \tr -> do
+    tr' <- removeFromTree path tr
+    return $ commitTree .~ ObjRef tr' $ c
 
 doUpdateCommit :: [Text] -> TreeEntry -> Commit -> IO Commit
 doUpdateCommit xs item c = do
diff --git a/Data/Git/Errors.hs b/Data/Git/Errors.hs
--- a/Data/Git/Errors.hs
+++ b/Data/Git/Errors.hs
@@ -21,6 +21,7 @@
                   | TreeBuilderWriteFailed
                   | TreeLookupFailed
                   | TreeCannotTraverseBlob
+                  | TreeEntryLookupFailed
                   | CommitCreateFailed
                   | CommitLookupFailed
                   | ReferenceCreateFailed
diff --git a/Data/Git/Internal.hs b/Data/Git/Internal.hs
--- a/Data/Git/Internal.hs
+++ b/Data/Git/Internal.hs
@@ -21,6 +21,8 @@
        , repositoryPtr
 
        , lookupObject'
+       , withObject
+       , withObjectPtr
 
        , module X )
        where
@@ -156,6 +158,21 @@
                           , _repoObj  = Just fptr }
 
   where doesNotExist = throwIO . RepositoryNotExist . toString
+
+withObject :: (Updatable a, Updatable b) => ObjRef a -> b -> (a -> IO c) -> IO c
+withObject objRef parent f = do
+  obj <- loadObject objRef parent
+  case obj of
+    Nothing   -> error "Cannot find Git object in repository"
+    Just obj' -> f obj'
+
+withObjectPtr :: (Updatable a, Updatable b)
+              => ObjRef a -> b -> (Ptr c -> IO d) -> IO d
+withObjectPtr objRef parent f =
+  withObject objRef parent $ \obj ->
+    case objectPtr obj of
+      Nothing     -> error "Cannot find Git object id"
+      Just objPtr -> withForeignPtr objPtr (f . castPtr)
 
 lookupObject'
   :: Oid -> Repository
diff --git a/Data/Git/Oid.hs b/Data/Git/Oid.hs
--- a/Data/Git/Oid.hs
+++ b/Data/Git/Oid.hs
@@ -42,6 +42,7 @@
 --   loadObject from the type class 'Updatable'.
 data ObjRef a = IdRef COid
               | ObjRef a
+              deriving Show
 
 wrapOidPtr :: Ptr C'git_oid -> IO (ObjRef a)
 wrapOidPtr = newForeignPtr_ >=> return . IdRef . COid
diff --git a/Data/Git/Repository.hs b/Data/Git/Repository.hs
--- a/Data/Git/Repository.hs
+++ b/Data/Git/Repository.hs
@@ -15,7 +15,12 @@
 
        , openRepository
        , createRepository
-       , openOrCreateRepository )
+       , openOrCreateRepository
+
+       , lookupObject'
+       , withObject
+       , withObjectPtr
+       )
        where
 
 import Data.Git.Internal
diff --git a/Data/Git/Tree.hs b/Data/Git/Tree.hs
--- a/Data/Git/Tree.hs
+++ b/Data/Git/Tree.hs
@@ -89,6 +89,34 @@
                        newBase repo (Stored coid) (Just obj)
                   , _treeContents = M.empty }
 
+doLookupTreeEntry :: [Text] -> Tree -> IO (Maybe TreeEntry)
+doLookupTreeEntry [] _ = throw TreeEntryLookupFailed
+doLookupTreeEntry (name:names) t = do
+  -- Lookup the current name in this tree.  If it doesn't exist, and there are
+  -- more names in the path and 'createIfNotExist' is True, create a new Tree
+  -- and descend into it.  Otherwise, if it exists we'll have @Just (TreeEntry
+  -- {})@, and if not we'll have Nothing.
+  y <- case M.lookup name (t^.treeContents) of
+    Nothing -> return Nothing
+    Just j  -> case j of
+      BlobEntry b mode -> do
+        bl <- loadObject b t
+        for bl $ \x -> return $ BlobEntry (ObjRef x) mode
+      TreeEntry t' -> do
+        tr <- loadObject t' t
+        for tr $ \x -> return $ TreeEntry (ObjRef x)
+
+  if null names
+    then return y
+    else
+      case y of
+        Just (BlobEntry {}) -> throw TreeCannotTraverseBlob
+        Just (TreeEntry (ObjRef t')) -> doLookupTreeEntry names t'
+        _ -> throw TreeEntryLookupFailed
+
+lookupTreeEntry :: FilePath -> Tree -> IO (Maybe TreeEntry)
+lookupTreeEntry = doLookupTreeEntry . splitPath
+
 withGitTree :: Updatable b
             => ObjRef Tree -> b -> (Ptr C'git_tree -> IO a) -> IO a
 withGitTree tref obj f =
@@ -150,7 +178,7 @@
     repo = fromMaybe (error "Repository invalid") $
                      t^.treeInfo.gitRepo.repoObj
 
-    insertObject :: (CStringable a, Updatable b)
+    insertObject :: (CStringable a, Updatable b, Show b)
                  => Ptr C'git_treebuilder -> a -> ObjRef b -> CUInt
                  -> IO (ObjRef b)
     insertObject builder key obj attrs = do
@@ -160,7 +188,7 @@
           oid <- objectId x
           case oid of
             Oid (COid y) -> return y
-            _ -> error "Unexpected"
+            _ -> error $ "Failed to determine object id for: " ++ show x
 
       withForeignPtr coid $ \coid' ->
         withCStringable key $ \name -> do
diff --git a/gitlib.cabal b/gitlib.cabal
--- a/gitlib.cabal
+++ b/gitlib.cabal
@@ -1,5 +1,5 @@
 Name:                gitlib
-Version:             0.4.0
+Version:             0.4.1
 Synopsis:            Higher-level types for working with hlibgit2
 Description:         Higher-level types for working with hlibgit2
 License-file:        LICENSE
@@ -38,7 +38,7 @@
   main-is: doctests.hs
   build-depends:
       base == 4.*
-    , directory >= 1.0 && < 1.2
+    , directory >= 1.0 && < 1.3
     , doctest   >= 0.8 && <= 0.10
     , filepath  >= 1.3 && < 1.4
   ghc-options: -Wall -Werror