diff --git a/lib_eio/workpool.ml b/lib_eio/workpool.ml
new file mode 100644
index 000000000..5102e0d3f
--- /dev/null
+++ b/lib_eio/workpool.ml
@@ -0,0 +1,144 @@
+type job = Pack : (unit -> 'a) * ('a, exn) Result.t Promise.u -> job
+
+type action =
+  | Process of job
+  | Quit of {
+      atomic: int Atomic.t;
+      target: int;
+      all_done: unit Promise.u;
+    }
+
+(* Worker: 1 domain/thread
+   m jobs per worker, n domains per workpool *)
+
+type t = {
+  (* The work queue *)
+  stream: action Stream.t;
+  (* Number of domains. Depending on settings, domains may run more than 1 job at a time. *)
+  domain_count: int;
+  (* True when [Workpool.terminate] has been called. *)
+  is_terminating: bool Atomic.t;
+  (* Resolved when the workpool begins terminating. *)
+  terminating: action Promise.t * action Promise.u;
+  (* Resolved when the workpool has terminated. *)
+  terminated: unit Promise.t * unit Promise.u;
+}
+
+(* This function is the core of workpool.ml.
+   Each worker recursively calls [loop ()] until the [terminating]
+   promise is resolved. Workers pull one job at a time from the Stream. *)
+let start_worker ~sw ~limit ~terminating stream =
+  let capacity = ref limit in
+  let condition = Condition.create () in
+  let run_job job w =
+    Fiber.fork ~sw (fun () ->
+        decr capacity;
+        Promise.resolve w
+          (try Ok (job ()) with
+           | exn -> Error exn);
+        incr capacity;
+        Condition.broadcast condition )
+  in
+  (* The main worker loop. *)
+  let rec loop () =
+    let action =
+      (* Peeking to minimize the chance of dropping jobs with [Fiber.first]. See Issue #558.
+         Issue #558 must be (will be) fixed before this module can be considered for merging. *)
+      match Promise.peek terminating with
+      | Some x -> x
+      | None -> Fiber.first (fun () -> Stream.take stream) (fun () -> Promise.await terminating)
+    in
+    match action with
+    | Process (Pack (job, w)) ->
+      (* We start the job right away. This also gives a chance to other domains
+         to start waiting on the Stream before the current thread blocks on [Stream.take] again. *)
+      run_job job w;
+      while !capacity = 0 do
+        Condition.await_no_mutex condition
+      done;
+      (loop [@tailcall]) ()
+    | Quit { atomic; target; all_done } ->
+      (* Wait until the completion of all of this worker's jobs. *)
+      while !capacity < limit do
+        Condition.await_no_mutex condition
+      done;
+      (* If we're the last worker terminating, resolve the promise. *)
+      if Atomic.fetch_and_add atomic 1 = target then Promise.resolve all_done ()
+  in
+  loop ()
+
+(* Start a new domain. The worker will need a switch, then we start the worker. *)
+let start_domain ~sw ~domain_mgr ~limit ~terminating ~transient stream =
+  let go () =
+    Domain_manager.run domain_mgr (fun () ->
+        Switch.run @@ fun sw -> start_worker ~sw ~limit ~terminating stream )
+  in
+  (* [transient] workpools run as daemons to not hold the user's switch from completing.
+     It's up to the user to hold the switch open (and thus, the workpool)
+     by blocking on the jobs issued to the workpool.
+     [Workpool.run] and [Workpool.run_exn] will block so this shouldn't be a problem.
+     Still, the user can call [Workpool.create] with [~transient:false] to
+     disable this behavior, in which case the user must call [Workpool.terminate]
+     to release the switch. *)
+  match transient with
+  | false -> Fiber.fork ~sw go
+  | true ->
+    Fiber.fork_daemon ~sw (fun () ->
+        go ();
+        `Stop_daemon )
+
+let create ~sw ~domain_count ~domain_concurrency ?(capacity = 0) ?(transient = true) domain_mgr =
+  if capacity < 0 then raise (Invalid_argument "Workpool capacity < 0");
+  let stream = Stream.create capacity in
+  let instance =
+    {
+      stream;
+      domain_count;
+      is_terminating = Atomic.make false;
+      terminating = Promise.create ();
+      terminated = Promise.create ();
+    }
+  in
+  let terminating = fst instance.terminating in
+  for _ = 1 to domain_count do
+    start_domain ~sw ~domain_mgr ~limit:domain_concurrency ~terminating ~transient stream
+  done;
+  instance
+
+let run_promise ~sw { stream; _ } f =
+  let p, w = Promise.create () in
+  Fiber.fork_promise ~sw (fun () ->
+      Stream.add stream (Process (Pack (f, w)));
+      Promise.await_exn p )
+
+let run { stream; _ } f =
+  let p, w = Promise.create () in
+  Stream.add stream (Process (Pack (f, w)));
+  Promise.await p
+
+let run_exn instance f =
+  match run instance f with
+  | Ok x -> x
+  | Error exn -> raise exn
+
+let terminate ~sw ({ terminating = _, w1; terminated = p2, w2; _ } as instance) =
+  if Atomic.compare_and_set instance.is_terminating false true
+  then (
+    (* Instruct workers to shutdown *)
+    Promise.resolve w1 (Quit { atomic = Atomic.make 1; target = instance.domain_count; all_done = w2 });
+    (* Reject all present and future queued jobs *)
+    Fiber.fork_daemon ~sw (fun () ->
+        while true do
+          match Stream.take instance.stream with
+          | Process (Pack (_, w)) -> Promise.resolve_error w (Failure "Workpool.terminate called")
+          | Quit _ -> ()
+        done;
+        `Stop_daemon );
+    (* Wait for all workers to have shutdown *)
+    Promise.await p2 )
+  else (* [Workpool.terminate] was called more than once. *)
+    Promise.await p2
+
+let is_terminating { terminating = p, _; _ } = Promise.is_resolved p
+
+let is_terminated { terminated = p, _; _ } = Promise.is_resolved p
diff --git a/lib_eio/workpool.mli b/lib_eio/workpool.mli
new file mode 100644
index 000000000..382e36f4d
--- /dev/null
+++ b/lib_eio/workpool.mli
@@ -0,0 +1,40 @@
+type t
+
+(** Creates a new workpool with [domain_count].
+
+    [domain_concurrency] is the maximum number of jobs that each domain can run at a time.
+
+    [capacity] (default: 0) is identical to the [Eio.Stream.create] capacity parameter.
+
+    [transient] (default: true). When true, the workpool will not block the [~sw] Switch from completing.
+      When false, you must call [terminate] to release the [~sw] Switch. *)
+val create :
+  sw:Switch.t ->
+  domain_count:int ->
+  domain_concurrency:int ->
+  ?capacity:int ->
+  ?transient:bool ->
+  #Domain_manager.t ->
+  t
+
+(** Run a job on this workpool. It is placed at the end of the queue. *)
+val run : t -> (unit -> 'a) -> ('a, exn) result
+
+(** Same as [run] but raises if the job failed. *)
+val run_exn : t -> (unit -> 'a) -> 'a
+
+(** Same as [run] but returns immediately, without blocking. *)
+val run_promise : sw:Switch.t -> t -> (unit -> 'a) -> ('a, exn) result Promise.t
+
+(** Waits for all running jobs to complete, then returns.
+    No new jobs are started, even if they were already enqueued.
+    To abort all running jobs instead of waiting for them, call [Switch.fail] on the Switch used to create this workpool *)
+val terminate : sw:Switch.t -> t -> unit
+
+(** Returns true if the [terminate] function has been called on this workpool.
+    Also returns true if the workpool has fully terminated. *)
+val is_terminating : t -> bool
+
+(** Returns true if the [terminate] function has been called on this workpool AND
+    the workpool has fully terminated (all running jobs have completed). *)
+val is_terminated : t -> bool