diff --git a/src/machinery.erl b/src/machinery.erl
index 6adeed8..2737f06 100644
--- a/src/machinery.erl
+++ b/src/machinery.erl
@@ -11,7 +11,9 @@
 -module(machinery).
 
 -type namespace()     :: atom().
+-type ref()           :: id() | {tag, tag()}.
 -type id()            :: binary().
+-type tag()           :: binary().
 -type args(T)         :: T.
 -type response(T)     :: T.
 
@@ -41,6 +43,8 @@
 
 -export_type([namespace/0]).
 -export_type([id/0]).
+-export_type([tag/0]).
+-export_type([ref/0]).
 -export_type([range/0]).
 -export_type([args/1]).
 -export_type([response/1]).
@@ -94,6 +98,7 @@
     {set_timer, timer()}                     |
     {set_timer, timer(), range()}            |
     {set_timer, timer(), range(), seconds()} |
+    {tag, tag()}                             |
     unset_timer                              |
     continue                                 |
     remove.
@@ -124,38 +129,38 @@ start(NS, ID, Args, Backend) ->
     {Module, Opts} = machinery_utils:get_backend(Backend),
     machinery_backend:start(Module, NS, ID, Args, Opts).
 
--spec call(namespace(), id(), args(_), backend(_)) ->
+-spec call(namespace(), ref(), args(_), backend(_)) ->
     {ok, response(_)} | {error, notfound}.
-call(NS, ID, Args, Backend) ->
-    call(NS, ID, {undefined, undefined, forward}, Args, Backend).
+call(NS, Ref, Args, Backend) ->
+    call(NS, Ref, {undefined, undefined, forward}, Args, Backend).
 
--spec call(namespace(), id(), range(), args(_), backend(_)) ->
+-spec call(namespace(), ref(), range(), args(_), backend(_)) ->
     {ok, response(_)} | {error, notfound}.
-call(NS, ID, Range, Args, Backend) ->
+call(NS, Ref, Range, Args, Backend) ->
     {Module, Opts} = machinery_utils:get_backend(Backend),
-    machinery_backend:call(Module, NS, ID, Range, Args, Opts).
+    machinery_backend:call(Module, NS, Ref, Range, Args, Opts).
 
--spec repair(namespace(), id(), args(_), backend(_)) ->
+-spec repair(namespace(), ref(), args(_), backend(_)) ->
     ok | {error, notfound | working}.
-repair(NS, ID, Args, Backend) ->
-    repair(NS, ID, {undefined, undefined, forward}, Args, Backend).
+repair(NS, Ref, Args, Backend) ->
+    repair(NS, Ref, {undefined, undefined, forward}, Args, Backend).
 
--spec repair(namespace(), id(), range(), args(_), backend(_)) ->
+-spec repair(namespace(), ref(), range(), args(_), backend(_)) ->
     ok | {error, notfound | working}.
-repair(NS, ID, Range, Args, Backend) ->
+repair(NS, Ref, Range, Args, Backend) ->
     {Module, Opts} = machinery_utils:get_backend(Backend),
-    machinery_backend:repair(Module, NS, ID, Range, Args, Opts).
+    machinery_backend:repair(Module, NS, Ref, Range, Args, Opts).
 
--spec get(namespace(), id(), backend(_)) ->
+-spec get(namespace(), ref(), backend(_)) ->
     {ok, machine(_, _)} | {error, notfound}.
-get(NS, ID, Backend) ->
-    get(NS, ID, {undefined, undefined, forward}, Backend).
+get(NS, Ref, Backend) ->
+    get(NS, Ref, {undefined, undefined, forward}, Backend).
 
--spec get(namespace(), id(), range(), backend(_)) ->
+-spec get(namespace(), ref(), range(), backend(_)) ->
     {ok, machine(_, _)} | {error, notfound}.
-get(NS, ID, Range, Backend) ->
+get(NS, Ref, Range, Backend) ->
     {Module, Opts} = machinery_utils:get_backend(Backend),
-    machinery_backend:get(Module, NS, ID, Range, Opts).
+    machinery_backend:get(Module, NS, Ref, Range, Opts).
 
 %% Internal API
 
diff --git a/src/machinery_backend.erl b/src/machinery_backend.erl
index cf4898d..a5e667e 100644
--- a/src/machinery_backend.erl
+++ b/src/machinery_backend.erl
@@ -13,6 +13,7 @@
 
 -type namespace()    :: machinery:namespace().
 -type id()           :: machinery:id().
+-type ref()          :: machinery:ref().
 -type range()        :: machinery:range().
 -type args()         :: machinery:args(_).
 -type backend_opts() :: machinery:backend_opts(_).
@@ -38,17 +39,17 @@
 start(Backend, Namespace, Id, Args, Opts) ->
     Backend:start(Namespace, Id, Args, Opts).
 
--spec call(backend(), namespace(), id(), range(), args(), backend_opts()) ->
+-spec call(backend(), namespace(), ref(), range(), args(), backend_opts()) ->
     {ok, machinery:response(_)} | {error, notfound}.
-call(Backend, Namespace, Id, Range, Args, Opts) ->
-    Backend:call(Namespace, Id, Range, Args, Opts).
+call(Backend, Namespace, Ref, Range, Args, Opts) ->
+    Backend:call(Namespace, Ref, Range, Args, Opts).
 
--spec repair(backend(), namespace(), id(), range(), args(), backend_opts()) ->
+-spec repair(backend(), namespace(), ref(), range(), args(), backend_opts()) ->
     ok | {error, notfound | working}.
-repair(Backend, Namespace, Id, Range, Args, Opts) ->
-    Backend:repair(Namespace, Id, Range, Args, Opts).
+repair(Backend, Namespace, Ref, Range, Args, Opts) ->
+    Backend:repair(Namespace, Ref, Range, Args, Opts).
 
--spec get(backend(), namespace(), id(), range(), backend_opts()) ->
+-spec get(backend(), namespace(), ref(), range(), backend_opts()) ->
     {ok, machinery:machine(_, _)} | {error, notfound}.
-get(Backend, Namespace, Id, Range, Opts) ->
-    Backend:get(Namespace, Id, Range, Opts).
+get(Backend, Namespace, Ref, Range, Opts) ->
+    Backend:get(Namespace, Ref, Range, Opts).
diff --git a/src/machinery_mg_backend.erl b/src/machinery_mg_backend.erl
index baaf349..803d755 100644
--- a/src/machinery_mg_backend.erl
+++ b/src/machinery_mg_backend.erl
@@ -9,6 +9,7 @@
 -include_lib("mg_proto/include/mg_proto_state_processing_thrift.hrl").
 
 -type namespace()       :: machinery:namespace().
+-type ref()             :: machinery:ref().
 -type id()              :: machinery:id().
 -type range()           :: machinery:range().
 -type args(T)           :: machinery:args(T).
@@ -120,12 +121,12 @@ start(NS, ID, Args, Opts) ->
             error({failed, NS, ID})
     end.
 
--spec call(namespace(), id(), range(), args(_), backend_opts()) ->
+-spec call(namespace(), ref(), range(), args(_), backend_opts()) ->
     {ok, response(_)} | {error, notfound}.
-call(NS, ID, Range, Args, Opts) ->
+call(NS, Ref, Range, Args, Opts) ->
     Client = get_client(Opts),
     Schema = get_schema(Opts),
-    Descriptor = {NS, ID, Range},
+    Descriptor = {NS, Ref, Range},
     CallArgs = marshal({schema, Schema, {args, call}}, Args),
     case machinery_mg_client:call(marshal(descriptor, Descriptor), CallArgs, Client) of
         {ok, Response} ->
@@ -135,15 +136,15 @@ call(NS, ID, Range, Args, Opts) ->
         {exception, #mg_stateproc_NamespaceNotFound{}} ->
             error({namespace_not_found, NS});
         {exception, #mg_stateproc_MachineFailed{}} ->
-            error({failed, NS, ID})
+            error({failed, NS, Ref})
     end.
 
--spec repair(namespace(), id(), range(), args(_), backend_opts()) ->
+-spec repair(namespace(), ref(), range(), args(_), backend_opts()) ->
     ok | {error, notfound | working}.
-repair(NS, ID, Range, Args, Opts) ->
+repair(NS, Ref, Range, Args, Opts) ->
     Client = get_client(Opts),
     Schema = get_schema(Opts),
-    Descriptor = {NS, ID, Range},
+    Descriptor = {NS, Ref, Range},
     CallArgs = marshal({schema, Schema, {args, repair}}, Args),
     case machinery_mg_client:repair(marshal(descriptor, Descriptor), CallArgs, Client) of
         {ok, ok} ->
@@ -156,12 +157,12 @@ repair(NS, ID, Range, Args, Opts) ->
             error({namespace_not_found, NS})
     end.
 
--spec get(namespace(), id(), range(), backend_opts()) ->
+-spec get(namespace(), ref(), range(), backend_opts()) ->
     {ok, machine(_, _)} | {error, notfound}.
-get(NS, ID, Range, Opts) ->
+get(NS, Ref, Range, Opts) ->
     Client = get_client(Opts),
     Schema = get_schema(Opts),
-    Descriptor = {NS, ID, Range},
+    Descriptor = {NS, Ref, Range},
     case machinery_mg_client:get_machine(marshal(descriptor, Descriptor), Client) of
         {ok, Machine} ->
             {ok, unmarshal({machine, Schema}, Machine)};
@@ -261,10 +262,10 @@ set_aux_state(NewState, _) ->
 %%         'history_range' = marshal(range, {undefined, undefined, forward})
 %%     };
 
-marshal(descriptor, {NS, ID, Range}) ->
+marshal(descriptor, {NS, Ref, Range}) ->
     #mg_stateproc_MachineDescriptor{
         'ns'        = marshal(namespace, NS),
-        'ref'       = {'id', marshal(id, ID)},
+        'ref'       = marshal(ref, Ref),
         'range'     = marshal(range, Range)
     };
 
@@ -334,9 +335,18 @@ marshal(timer, {deadline, V}) ->
 marshal(namespace, V) ->
     marshal(atom, V);
 
+marshal(ref, V) when is_binary(V) ->
+    {id, marshal(id, V)};
+
+marshal(ref, {tag, V}) ->
+    {tag, marshal(tag, V)};
+
 marshal(id, V) ->
     marshal(string, V);
 
+marshal(tag, V) ->
+    marshal(string, V);
+
 marshal(event_id, V) ->
     marshal(integer, V);
 
@@ -417,6 +427,13 @@ apply_action(continue, CA) ->
 apply_action(remove, CA) ->
     CA#mg_stateproc_ComplexAction{
         remove = #mg_stateproc_RemoveAction{}
+    };
+
+apply_action({tag, Tag}, CA) ->
+    CA#mg_stateproc_ComplexAction{
+        tag = #mg_stateproc_TagAction{
+            tag = marshal(tag, Tag)
+        }
     }.
 
 %%