diff --git a/sqlexpr_concurrency.ml b/sqlexpr_concurrency.ml
index 91b02fb..0da106a 100644
--- a/sqlexpr_concurrency.ml
+++ b/sqlexpr_concurrency.ml
@@ -1,5 +1,3 @@
-(** Concurrency monad. *)
-
 module type THREAD =
 sig
   type 'a t
diff --git a/sqlexpr_concurrency.mli b/sqlexpr_concurrency.mli
new file mode 100644
index 0000000..edf518b
--- /dev/null
+++ b/sqlexpr_concurrency.mli
@@ -0,0 +1,37 @@
+(** Concurrency monad. *)
+
+(** The THREAD monad. *)
+module type THREAD =
+sig
+  type 'a t
+  val return : 'a -> 'a t
+  val bind : 'a t -> ('a -> 'b t) -> 'b t
+  val fail : exn -> 'a t
+  val catch : (unit -> 'a t) -> (exn -> 'a t) -> 'a t
+  val finalize : (unit -> 'a t) -> (unit -> unit t) -> 'a t
+  val sleep : float -> unit t
+  val auto_yield : float -> unit -> unit t
+
+  type mutex
+
+  (** Create a recursive mutex that can be locked recursively by the same
+    * thread; i.e., unlike a regular mutex,
+    * [with_lock m (fun () -> ... with_lock m (fun () -> ... ))]
+    * will not block. *)
+  val create_recursive_mutex : unit -> mutex
+
+  (* [with_lock m f] blocks until the [m] mutex can be locked, runs [f ()] and
+   * unlocks the mutex (also if [f ()] raises an exception) *)
+  val with_lock : mutex -> (unit -> 'a t) -> 'a t
+end
+
+(** Identity concurrency monad. Note that [Id.mutex] is a dummy type that
+  * doesn't actually work like a mutex (i.e., [Id.with_lock m f] is equivalent
+  * to [f ()]. This is so because n ocaml-sqlexpr's context [Sqlite] handles
+  * can only be used from the thread where they were created, so there's no
+  * need for mutual exclusion because trying to use the same handle from
+  * different threads would be an error anyway. *)
+module Id : THREAD with type 'a t = 'a
+
+(** Lwt concurrency monad. *)
+module Lwt : THREAD with type 'a t = 'a Lwt.t