diff --git a/Annex/MetaData.hs b/Annex/MetaData.hs
index d8d8177c46..6642c36150 100644
--- a/Annex/MetaData.hs
+++ b/Annex/MetaData.hs
@@ -63,7 +63,7 @@ parseModMeta p = case lastMaybe f of
 	Just '+' -> AddMeta <$> mkMetaField f' <*> v
 	Just '-' -> DelMeta <$> mkMetaField f' <*> (Just <$> v)
 	Just '?' -> MaybeSetMeta <$> mkMetaField f' <*> v
-	_ -> SetMeta <$> mkMetaField f <*> v
+	_ -> SetMeta <$> mkMetaField f <*> (S.singleton <$> v)
   where
 	(f, sv) = separate (== '=') p
 	f' = beginning f
diff --git a/CHANGELOG b/CHANGELOG
index ca2a417eab..bccf70fa1e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -5,6 +5,8 @@ git-annex (6.20160726) UNRELEASED; urgency=medium
     This should be easier to parse than the old format, which mixed up
     metadata fields with other keys in the json object.
     Any consumers of the old format will need to be updated.
+  * Added metadata --batch option, which allows getting, setting, deleting,
+    and modifying metadata for multiple files/keys.
   * Added --branch option to copy, drop, fsck, get, metadata, mirror, move,
     and whereis commands. This option makes git-annex operate on files that
     are included in a specified branch (or other treeish).
diff --git a/Command/MetaData.hs b/Command/MetaData.hs
index d33372d0b0..4233c56a76 100644
--- a/Command/MetaData.hs
+++ b/Command/MetaData.hs
@@ -10,9 +10,12 @@ module Command.MetaData where
 import Command
 import Annex.MetaData
 import Logs.MetaData
+import Annex.WorkTree
 import Messages.JSON (JSONActionItem(..))
+import Types.Messages
 
 import qualified Data.Set as S
+import qualified Data.Map as M
 import qualified Data.Text as T
 import qualified Data.ByteString.Lazy.UTF8 as BU
 import Data.Time.Clock.POSIX
@@ -28,6 +31,7 @@ data MetaDataOptions = MetaDataOptions
 	{ forFiles :: CmdParams
 	, getSet :: GetSet
 	, keyOptions :: Maybe KeyOptions
+	, batchOption :: BatchMode
 	}
 
 data GetSet = Get MetaField | GetAll | Set [ModMeta]
@@ -37,6 +41,7 @@ optParser desc = MetaDataOptions
 	<$> cmdParams desc
 	<*> ((Get <$> getopt) <|> (Set <$> some modopts) <|> pure GetAll)
 	<*> optional (parseKeyOptions False)
+	<*> parseBatchOption
   where
 	getopt = option (eitherReader mkMetaField)
 		( long "get" <> short 'g' <> metavar paramField
@@ -62,15 +67,21 @@ optParser desc = MetaDataOptions
 seek :: MetaDataOptions -> CommandSeek
 seek o = do
 	now <- liftIO getPOSIXTime
-	let seeker = case getSet o of
-		Get _ -> withFilesInGit
-		GetAll -> withFilesInGit
-		Set _ -> withFilesInGitNonRecursive
-			"Not recursively setting metadata. Use --force to do that."
-	withKeyOptions (keyOptions o) False
-		(startKeys now o)
-		(seeker $ whenAnnexed $ start now o)
-		(forFiles o)
+	case batchOption o of
+		NoBatch -> do
+			let seeker = case getSet o of
+				Get _ -> withFilesInGit
+				GetAll -> withFilesInGit
+				Set _ -> withFilesInGitNonRecursive
+					"Not recursively setting metadata. Use --force to do that."
+			withKeyOptions (keyOptions o) False
+				(startKeys now o)
+				(seeker $ whenAnnexed $ start now o)
+				(forFiles o)
+		Batch -> withOutputType $ \ot -> case ot of
+			JSONOutput -> batchInput parseJSONInput $
+				commandAction . startBatch now
+			_ -> error "--batch is currently only supported in --json mode"
 
 start :: POSIXTime -> MetaDataOptions -> FilePath -> Key -> CommandStart
 start now o file k = startKeys now o k (mkActionItem afile)
@@ -128,14 +139,37 @@ instance FromJSON MetaDataFields where
 fieldsField :: T.Text
 fieldsField = T.pack "fields"
 
-parseJSONInput :: String -> Maybe (Either FilePath Key, MetaData)
+parseJSONInput :: String -> Either String (Either FilePath Key, MetaData)
 parseJSONInput i = do
-	v <- decode (BU.fromString i)
+	v <- eitherDecode (BU.fromString i)
 	let m = case itemAdded v of
 		Nothing -> emptyMetaData
 		Just (MetaDataFields m') -> m'
-	let keyfile = case (itemKey v, itemFile v) of
-		(Just k, _) -> Right k
-		(Nothing, Just f) -> Left f
-		(Nothing, Nothing) -> error "JSON input is missing either file or key"
-	return (keyfile, m)
+	case (itemKey v, itemFile v) of
+		(Just k, _) -> Right (Right k, m)
+		(Nothing, Just f) -> Right (Left f, m)
+		(Nothing, Nothing) -> Left "JSON input is missing either file or key"
+
+startBatch :: POSIXTime -> (Either FilePath Key, MetaData) -> CommandStart
+startBatch now (i, (MetaData m)) = case i of
+	Left f -> do
+		mk <- lookupFile f
+		case mk of
+			Just k -> go k (mkActionItem (Just f))
+			Nothing -> error $ "not an annexed file: " ++ f
+	Right k -> go k (mkActionItem k)
+  where
+	go k ai = do
+		showStart' "metadata" k ai
+		let o = MetaDataOptions
+			{ forFiles = []
+			, getSet = if MetaData m == emptyMetaData
+				then GetAll
+				else Set $ map mkModMeta (M.toList m)
+			, keyOptions = Nothing
+			, batchOption = NoBatch
+			}
+		next $ perform now o k
+	mkModMeta (f, s)
+		| S.null s = DelMeta f Nothing
+		| otherwise = SetMeta f s
diff --git a/Messages.hs b/Messages.hs
index 63f5b10bb2..339823d42b 100644
--- a/Messages.hs
+++ b/Messages.hs
@@ -42,6 +42,7 @@ module Messages (
 	commandProgressDisabled,
 	outputMessage,
 	implicitMessage,
+	withOutputType,
 ) where
 
 import System.Log.Logger
diff --git a/Types/MetaData.hs b/Types/MetaData.hs
index a62dd7ed08..9e153ffb9a 100644
--- a/Types/MetaData.hs
+++ b/Types/MetaData.hs
@@ -204,8 +204,11 @@ emptyMetaData = MetaData M.empty
 {- Can be used to set a value, or to unset it, depending on whether
  - the MetaValue has CurrentlySet or not. -}
 updateMetaData :: MetaField -> MetaValue -> MetaData -> MetaData
-updateMetaData f v (MetaData m) = MetaData $
-	M.insertWith' S.union f (S.singleton v) m
+updateMetaData f v = updateMetaData' f (S.singleton v)
+
+updateMetaData' :: MetaField -> S.Set MetaValue -> MetaData -> MetaData
+updateMetaData' f s (MetaData m) = MetaData $
+	M.insertWith' S.union f s m
 
 {- New metadata overrides old._-}
 unionMetaData :: MetaData -> MetaData -> MetaData
@@ -247,7 +250,7 @@ data ModMeta
 	| DelMeta MetaField (Maybe MetaValue)
 	-- ^ delete value of a field. With Just, only that specific value
 	-- is deleted; with Nothing, all current values are deleted.
-	| SetMeta MetaField MetaValue
+	| SetMeta MetaField (S.Set MetaValue)
 	-- ^ removes any existing values
 	| MaybeSetMeta MetaField MetaValue
 	-- ^ set when field has no existing value
@@ -262,7 +265,7 @@ modMeta _ (DelMeta f (Just oldv)) =
 	updateMetaData f (unsetMetaValue oldv) emptyMetaData
 modMeta m (DelMeta f Nothing) = MetaData $ M.singleton f $
 	S.fromList $ map unsetMetaValue $ S.toList $ currentMetaDataValues f m
-modMeta m (SetMeta f v) = updateMetaData f v $
+modMeta m (SetMeta f s) = updateMetaData' f s $
 	foldr (updateMetaData f) emptyMetaData $
 		map unsetMetaValue $ S.toList $ currentMetaDataValues f m
 modMeta m (MaybeSetMeta f v)
diff --git a/doc/git-annex-metadata.mdwn b/doc/git-annex-metadata.mdwn
index b4e7900808..fde4443dc7 100644
--- a/doc/git-annex-metadata.mdwn
+++ b/doc/git-annex-metadata.mdwn
@@ -17,6 +17,11 @@ metadata.
 
 When run without any -s or -t parameters, displays the current metadata.
 
+Each metadata field has its own "field-lastchanged" metadata, which
+contains the date the field was last changed. Unlike other metadata,
+this cannot be directly modified by this command. It is updated
+automatically.
+
 # OPTIONS
 
 * `-g field` / `--get field`
@@ -66,21 +71,6 @@ When run without any -s or -t parameters, displays the current metadata.
   The [[git-annex-matching-options]](1)
   can be used to specify files to act on.
 
-* `--json`
-
-  Enable JSON output. This is intended to be parsed by programs that use
-  git-annex. Each line of output is a JSON object.
-
-  The format of the JSON objects changed in git-annex version 6.20160726.
-
-  Example of the new format:
-
-	{"command":"metadata","file":"foo","key":"...","fields":{"author":["bar"],...},"note":"...","success":true}
-
-  Example of the old format, which lacks the inner fields object:
-
-	{"command":"metadata","file":"foo","key":"...","author":["bar"],...,"note":"...","success":true}
-
 * `--all`
 
   Specify instead of a file to get/set metadata on all known keys.
@@ -99,6 +89,55 @@ When run without any -s or -t parameters, displays the current metadata.
 
   Specify instead of a file to get/set metadata of the specified key.
 
+* `--json`
+
+  Enable JSON output (and input). Each line is a JSON object.
+
+  The format of the JSON objects changed in git-annex version 6.20160726.
+
+  Example of the new format:
+
+	{"command":"metadata","file":"foo","key":"...","fields":{"author":["bar"],...},"note":"...","success":true}
+
+  Example of the old format, which lacks the inner fields object:
+
+	{"command":"metadata","file":"foo","key":"...","author":["bar"],...,"note":"...","success":true}
+
+* `--batch`
+
+  Enables batch mode, which can be used to both get, store, and unset
+  metadata for multiple files or keys.
+
+  Batch currently only supports JSON input. So, you must
+  enable `--json` along with `--batch`.
+
+  In batch mode, git-annex reads lines from stdin, which contain
+  JSON objects. It replies to each input with an output JSON object.
+
+  The format of the JSON sent to git-annex can be the same as the JSON that
+  it outputs. Or, a simplified version. Only the "file" (or "key") field
+  is actually necessary.
+
+  For example, to get the current metadata of file foo:
+
+	{"file":"foo"}
+
+  To get the current metadata of the key k:
+	
+	{"key":"k"}
+
+  Any metadata fields included in the JSON object will be stored,
+  replacing whatever values the fields had before.
+  To unset a field, include it with an empty list of values.
+
+  To change the author of file foo to bar:
+
+	{"file":"foo","fields":{"author":["bar"]}}
+
+  To remove the author of file foo:
+
+	{"file":"foo","fields":{"author":[]}}
+
 # EXAMPLES
 
 To set some tags on a file and also its author:
diff --git a/doc/todo/ability_to_set_metadata_from_json.mdwn b/doc/todo/ability_to_set_metadata_from_json.mdwn
index ee4d68823a..1129650a0f 100644
--- a/doc/todo/ability_to_set_metadata_from_json.mdwn
+++ b/doc/todo/ability_to_set_metadata_from_json.mdwn
@@ -1,6 +1,13 @@
-I can export metadata to JSON format, which is nice as this can now be read into any other tool and manipulated. But I cannot find a way to set the metadata from JSON and so I am left to figure out what changes need to be made via the g-a interface to get to the desired state, and that is hard to get right.
+I can export metadata to JSON format, which is nice as this can now be read
+into any other tool and manipulated. But I cannot find a way to set the
+metadata from JSON and so I am left to figure out what changes need to be
+made via the g-a interface to get to the desired state, and that is hard to
+get right.
 
-Maybe g-a metadata could grow an import-json function which would set (overwrite) the metadata for the given file(s) from JSON input.
+Maybe g-a metadata could grow an import-json function which would set
+(overwrite) the metadata for the given file(s) from JSON input.
 
 Thanks,
 -m
+
+> [[done]] via `git annex metadata --batch --json` --[[Joey]]
diff --git a/doc/todo/metadata_--batch.mdwn b/doc/todo/metadata_--batch.mdwn
index 2680dfd2b8..b65dc3ef58 100644
--- a/doc/todo/metadata_--batch.mdwn
+++ b/doc/todo/metadata_--batch.mdwn
@@ -1 +1,3 @@
 [[!meta author=yoh]]
+
+> [[done]] (using json input) --[[Joey]]