diff --git a/src/core/fifo_pool.ml b/src/core/fifo_pool.ml
index df4837c2..7646ca11 100644
--- a/src/core/fifo_pool.ml
+++ b/src/core/fifo_pool.ml
@@ -6,11 +6,11 @@ let k_storage = Task_local_storage.Private_.Storage.k_storage
 
 type task_full =
   | T_start of {
-      ls: Task_local_storage.storage;
+      ls: Task_local_storage.storage ref;
       f: task;
     }
   | T_resume : {
-      ls: Task_local_storage.storage;
+      ls: Task_local_storage.storage ref;
       k: 'a -> unit;
       x: 'a;
     }
@@ -30,23 +30,22 @@ let schedule_ (self : state) (task : task_full) : unit =
   try Bb_queue.push self.q task with Bb_queue.Closed -> raise Shutdown
 
 type around_task = AT_pair : (t -> 'a) * (t -> 'a -> unit) -> around_task
-type worker_state = { cur_ls: Task_local_storage.storage ref }
+type worker_state = { mutable cur_ls: Task_local_storage.storage ref option }
 
 let k_worker_state : worker_state option ref TLS.key =
   TLS.new_key (fun () -> ref None)
 
 let worker_thread_ (self : state) (runner : t) ~on_exn ~around_task : unit =
-  let w = { cur_ls = ref Task_local_storage.Private_.Storage.dummy } in
+  let w = { cur_ls = None } in
   TLS.get k_worker_state := Some w;
-  TLS.set k_storage (Some w.cur_ls);
   TLS.get Runner.For_runner_implementors.k_cur_runner := Some runner;
 
   let (AT_pair (before_task, after_task)) = around_task in
 
   let on_suspend () =
     match !(TLS.get k_worker_state) with
-    | None -> assert false
-    | Some w -> !(w.cur_ls)
+    | Some { cur_ls = Some ls; _ } -> ls
+    | _ -> assert false
   in
   let run_another_task ls task' = schedule_ self @@ T_start { f = task'; ls } in
   let resume ls k res = schedule_ self @@ T_resume { ls; k; x = res } in
@@ -56,7 +55,8 @@ let worker_thread_ (self : state) (runner : t) ~on_exn ~around_task : unit =
       match task with
       | T_start { ls; _ } | T_resume { ls; _ } -> ls
     in
-    w.cur_ls := ls;
+    w.cur_ls <- Some ls;
+    TLS.set k_storage (Some ls);
     let _ctx = before_task runner in
 
     (* run the task now, catching errors, handling effects *)
@@ -74,7 +74,8 @@ let worker_thread_ (self : state) (runner : t) ~on_exn ~around_task : unit =
        let bt = Printexc.get_raw_backtrace () in
        on_exn e bt);
     after_task runner _ctx;
-    w.cur_ls := Task_local_storage.Private_.Storage.dummy
+    w.cur_ls <- None;
+    TLS.set k_storage None
   in
 
   let main_loop () =
diff --git a/src/core/immediate_runner.ml b/src/core/immediate_runner.ml
index 9412fd35..4e15c434 100644
--- a/src/core/immediate_runner.ml
+++ b/src/core/immediate_runner.ml
@@ -4,10 +4,8 @@ include Runner
 (* convenient alias *)
 let k_ls = Task_local_storage.Private_.Storage.k_storage
 
-let run_async_ ~ls f =
-  let cur_ls = ref ls in
+let run_async_ ~ls:cur_ls f =
   TLS.set k_ls (Some cur_ls);
-  cur_ls := ls;
   try
     let x = f () in
     TLS.set k_ls None;
diff --git a/src/core/moonpool.mli b/src/core/moonpool.mli
index 92ed0dd5..df09d409 100644
--- a/src/core/moonpool.mli
+++ b/src/core/moonpool.mli
@@ -27,14 +27,14 @@ val start_thread_on_some_domain : ('a -> unit) -> 'a -> Thread.t
     to run all the various threads needed in an application (timers, event loops, etc.) *)
 
 val run_async :
-  ?ls:Task_local_storage.storage -> Runner.t -> (unit -> unit) -> unit
+  ?ls:Task_local_storage.storage ref -> Runner.t -> (unit -> unit) -> unit
 (** [run_async runner task] schedules the task to run
   on the given runner. This means [task()] will be executed
   at some point in the future, possibly in another thread.
   @since 0.5 *)
 
 val run_wait_block :
-  ?ls:Task_local_storage.storage -> Runner.t -> (unit -> 'a) -> 'a
+  ?ls:Task_local_storage.storage ref -> Runner.t -> (unit -> 'a) -> 'a
 (** [run_wait_block runner f] schedules [f] for later execution
     on the runner, like {!run_async}.
     It then blocks the current thread until [f()] is done executing,
diff --git a/src/core/runner.ml b/src/core/runner.ml
index 360ec6ba..391fdcd9 100644
--- a/src/core/runner.ml
+++ b/src/core/runner.ml
@@ -3,7 +3,7 @@ module TLS = Thread_local_storage_
 type task = unit -> unit
 
 type t = {
-  run_async: ls:Task_local_storage.storage -> task -> unit;
+  run_async: ls:Task_local_storage.storage ref -> task -> unit;
   shutdown: wait:bool -> unit -> unit;
   size: unit -> int;
   num_tasks: unit -> int;
@@ -11,8 +11,9 @@ type t = {
 
 exception Shutdown
 
-let[@inline] run_async ?(ls = Task_local_storage.Private_.Storage.create ())
-    (self : t) f : unit =
+let[@inline] run_async
+    ?(ls = ref @@ Task_local_storage.Private_.Storage.create ()) (self : t) f :
+    unit =
   self.run_async ~ls f
 
 let[@inline] shutdown (self : t) : unit = self.shutdown ~wait:true ()
diff --git a/src/core/runner.mli b/src/core/runner.mli
index 331e8b50..577a4b39 100644
--- a/src/core/runner.mli
+++ b/src/core/runner.mli
@@ -33,14 +33,15 @@ val shutdown_without_waiting : t -> unit
 
 exception Shutdown
 
-val run_async : ?ls:Task_local_storage.storage -> t -> task -> unit
+val run_async : ?ls:Task_local_storage.storage ref -> t -> task -> unit
 (** [run_async pool f] schedules [f] for later execution on the runner
     in one of the threads. [f()] will run on one of the runner's
     worker threads/domains.
     @param ls if provided, run the task with this initial local storage
     @raise Shutdown if the runner was shut down before [run_async] was called. *)
 
-val run_wait_block : ?ls:Task_local_storage.storage -> t -> (unit -> 'a) -> 'a
+val run_wait_block :
+  ?ls:Task_local_storage.storage ref -> t -> (unit -> 'a) -> 'a
 (** [run_wait_block pool f] schedules [f] for later execution
     on the pool, like {!run_async}.
     It then blocks the current thread until [f()] is done executing,
@@ -60,7 +61,7 @@ module For_runner_implementors : sig
     size:(unit -> int) ->
     num_tasks:(unit -> int) ->
     shutdown:(wait:bool -> unit -> unit) ->
-    run_async:(ls:Task_local_storage.storage -> task -> unit) ->
+    run_async:(ls:Task_local_storage.storage ref -> task -> unit) ->
     unit ->
     t
   (** Create a new runner.
diff --git a/src/core/ws_pool.ml b/src/core/ws_pool.ml
index 956dd9ce..367fbae2 100644
--- a/src/core/ws_pool.ml
+++ b/src/core/ws_pool.ml
@@ -18,11 +18,11 @@ type around_task = AT_pair : (t -> 'a) * (t -> 'a -> unit) -> around_task
 
 type task_full =
   | T_start of {
-      ls: Task_local_storage.storage;
+      ls: Task_local_storage.storage ref;
       f: task;
     }
   | T_resume : {
-      ls: Task_local_storage.storage;
+      ls: Task_local_storage.storage ref;
       k: 'a -> unit;
       x: 'a;
     }
@@ -32,7 +32,7 @@ type worker_state = {
   pool_id_: Id.t;  (** Unique per pool *)
   mutable thread: Thread.t;
   q: task_full WSQ.t;  (** Work stealing queue *)
-  cur_ls: Task_local_storage.storage ref;  (** Task storage *)
+  mutable cur_ls: Task_local_storage.storage ref option;  (** Task storage *)
   rng: Random.State.t;
 }
 (** State for a given worker. Only this worker is
@@ -120,17 +120,14 @@ let run_task_now_ (self : state) ~runner ~(w : worker_state) (task : task_full)
     | T_start { ls; _ } | T_resume { ls; _ } -> ls
   in
 
-  w.cur_ls := ls;
+  w.cur_ls <- Some ls;
+  TLS.set k_storage (Some ls);
   let _ctx = before_task runner in
 
-  let[@inline] on_suspend () =
-    let w =
-      match find_current_worker_ () with
-      | Some w -> w
-      | None -> assert false
-    in
-    let ls = !(w.cur_ls) in
-    ls
+  let[@inline] on_suspend () : _ ref =
+    match find_current_worker_ () with
+    | Some { cur_ls = Some w; _ } -> w
+    | _ -> assert false
   in
 
   let run_another_task ls (task' : task) =
@@ -139,7 +136,7 @@ let run_task_now_ (self : state) ~runner ~(w : worker_state) (task : task_full)
       | Some w when Id.equal w.pool_id_ self.id_ -> Some w
       | _ -> None
     in
-    let ls' = Task_local_storage.Private_.Storage.copy ls in
+    let ls' = ref @@ Task_local_storage.Private_.Storage.copy !ls in
     schedule_task_ self ~w @@ T_start { ls = ls'; f = task' }
   in
 
@@ -168,7 +165,8 @@ let run_task_now_ (self : state) ~runner ~(w : worker_state) (task : task_full)
      self.on_exn e bt);
 
   after_task runner _ctx;
-  w.cur_ls := Task_local_storage.Private_.Storage.dummy
+  w.cur_ls <- None;
+  TLS.set k_storage None
 
 let run_async_ (self : state) ~ls (f : task) : unit =
   let w = find_current_worker_ () in
@@ -291,7 +289,7 @@ type ('a, 'b) create_args =
 (** Arguments used in {!create}. See {!create} for explanations. *)
 
 let dummy_task_ : task_full =
-  T_start { f = ignore; ls = Task_local_storage.Private_.Storage.dummy }
+  T_start { f = ignore; ls = ref Task_local_storage.Private_.Storage.dummy }
 
 let create ?(on_init_thread = default_thread_init_exit_)
     ?(on_exit_thread = default_thread_init_exit_) ?(on_exn = fun _ _ -> ())
@@ -318,7 +316,7 @@ let create ?(on_init_thread = default_thread_init_exit_)
           thread = dummy;
           q = WSQ.create ~dummy:dummy_task_ ();
           rng = Random.State.make [| i |];
-          cur_ls = ref Task_local_storage.Private_.Storage.dummy;
+          cur_ls = None;
         })
   in
 
@@ -360,7 +358,7 @@ let create ?(on_init_thread = default_thread_init_exit_)
       let thread = Thread.self () in
       let t_id = Thread.id thread in
       on_init_thread ~dom_id:dom_idx ~t_id ();
-      TLS.set k_storage (Some w.cur_ls);
+      TLS.set k_storage None;
 
       (* set thread name *)
       Option.iter
diff --git a/src/fib/fiber.ml b/src/fib/fiber.ml
index d8390a2e..73c82ee4 100644
--- a/src/fib/fiber.ml
+++ b/src/fib/fiber.ml
@@ -16,6 +16,7 @@ module Private_ = struct
     state: 'a state A.t;  (** Current state in the lifetime of the fiber *)
     res: 'a Fut.t;
     runner: Runner.t;
+    ls: Task_local_storage.storage ref;
   }
 
   and 'a state =
@@ -28,12 +29,18 @@ module Private_ = struct
 
   and children = any FM.t
   and any = Any : _ t -> any [@@unboxed]
+  and nursery = Nursery : _ t -> nursery [@@unboxed]
 
   (** Key to access the current fiber. *)
   let k_current_fiber : any option Task_local_storage.key =
     Task_local_storage.new_key ~init:(fun () -> None) ()
 
   let[@inline] get_cur () : any option = Task_local_storage.get k_current_fiber
+
+  let[@inline] is_closed (self : _ t) =
+    match A.get self.state with
+    | Alive _ -> false
+    | Terminating_or_done _ -> true
 end
 
 include Private_
@@ -44,6 +51,9 @@ let[@inline] is_done self = Fut.is_done self.res
 let[@inline] is_success self = Fut.is_success self.res
 let[@inline] is_cancelled self = Fut.is_failed self.res
 let[@inline] on_result (self : _ t) f = Fut.on_result self.res f
+let[@inline] await self = Fut.await self.res
+let[@inline] wait_block self = Fut.wait_block self.res
+let[@inline] wait_block_exn self = Fut.wait_block_exn self.res
 
 (** Resolve [promise] once [children] are all done *)
 let resolve_once_children_are_done_ ~children ~promise
@@ -92,6 +102,53 @@ let rec resolve_as_failed_ : type a. a t -> Exn_bt.t -> unit =
 and cancel_children_ ebt ~children : unit =
   FM.iter (fun _ (Any f) -> resolve_as_failed_ f ebt) children
 
+type cancel_handle = int
+
+let add_on_cancel (self : _ t) cb : cancel_handle =
+  let h = ref 0 in
+  while
+    match A.get self.state with
+    | Alive { children; cancel_id; on_cancel } as old ->
+      let new_st =
+        Alive
+          {
+            children;
+            cancel_id = cancel_id + 1;
+            on_cancel = Int_map.add cancel_id cb on_cancel;
+          }
+      in
+      if A.compare_and_set self.state old new_st then (
+        h := cancel_id;
+        false
+      ) else
+        true
+    | Terminating_or_done r ->
+      (match A.get r with
+      | Error ebt -> cb ebt
+      | Ok _ -> ());
+      false
+  do
+    ()
+  done;
+  !h
+
+let remove_on_cancel (self : _ t) h =
+  while
+    match A.get self.state with
+    | Alive ({ on_cancel; _ } as alive) as old ->
+      let new_st =
+        Alive { alive with on_cancel = Int_map.remove h on_cancel }
+      in
+      not (A.compare_and_set self.state old new_st)
+    | Terminating_or_done _ -> false
+  do
+    ()
+  done
+
+let with_cancel_callback (self : _ t) cb (k : unit -> 'a) : 'a =
+  let h = add_on_cancel self cb in
+  Fun.protect k ~finally:(fun () -> remove_on_cancel self h)
+
 (** Successfully resolve the fiber *)
 let resolve_ok_ (self : 'a t) (r : 'a) : unit =
   let r = A.make @@ Ok r in
@@ -156,26 +213,28 @@ let add_child_ ~protect (self : _ t) (child : _ t) =
     ()
   done
 
-let spawn_ ~ls ~on (f : _ -> 'a) : 'a t =
+let create_ ~ls ~runner () : 'a t =
   let id = Handle.generate_fresh () in
   let res, _promise = Fut.make () in
-  let fib =
-    {
-      state =
-        A.make
-        @@ Alive
-             { children = FM.empty; on_cancel = Int_map.empty; cancel_id = 0 };
-      id;
-      res;
-      runner = on;
-    }
-  in
+  {
+    state =
+      A.make
+      @@ Alive { children = FM.empty; on_cancel = Int_map.empty; cancel_id = 0 };
+    id;
+    res;
+    runner;
+    ls;
+  }
+
+let spawn_ ~ls (Nursery n) (f : nursery -> 'a) : 'a t =
+  if is_closed n then failwith "spawn: nursery is closed";
+  let fib = create_ ~ls ~runner:n.runner () in
 
   let run () =
     (* make sure the fiber is accessible from inside itself *)
     Task_local_storage.set k_current_fiber (Some (Any fib));
     try
-      let res = f () in
+      let res = f (Nursery fib) in
       resolve_ok_ fib res
     with exn ->
       let bt = Printexc.get_raw_backtrace () in
@@ -183,90 +242,64 @@ let spawn_ ~ls ~on (f : _ -> 'a) : 'a t =
       resolve_as_failed_ fib ebt
   in
 
-  Runner.run_async ?ls on run;
+  Runner.run_async ~ls n.runner run;
 
   fib
 
-let[@inline] spawn_top ~on f : _ t = spawn_ ~ls:None ~on f
+let spawn (Nursery n) ?(protect = true) f : _ t =
+  (* spawn [f()] with a copy of our local storage *)
+  let ls = ref (Task_local_storage.Private_.Storage.copy !(n.ls)) in
+  let child = spawn_ ~ls (Nursery n) f in
+  add_child_ ~protect n child;
+  child
+
+let[@inline] spawn_ignore n ?protect f : unit =
+  ignore (spawn n ?protect f : _ t)
+
+module Nursery = struct
+  type t = nursery
+
+  let[@inline] await (Nursery n) : unit =
+    ignore (await n);
+    ()
+
+  let cancel_with (Nursery n) ebt : unit = resolve_as_failed_ n ebt
+
+  let with_create_top ~on () f =
+    let n =
+      create_
+        ~ls:(ref @@ Task_local_storage.Private_.Storage.create ())
+        ~runner:on ()
+    in
+    Fun.protect ~finally:(fun () -> resolve_ok_ n ()) (fun () -> f (Nursery n))
+
+  let with_create_sub ~protect (Nursery parent : t) f =
+    let n =
+      create_
+        ~ls:(ref @@ Task_local_storage.Private_.Storage.copy !(parent.ls))
+        ~runner:parent.runner ()
+    in
+    add_child_ ~protect parent n;
+    Fun.protect ~finally:(fun () -> resolve_ok_ n ()) (fun () -> f (Nursery n))
+
+  let[@inline] with_cancel_callback (Nursery self) cb f =
+    with_cancel_callback self cb f
+end
 
 let[@inline] self () : any =
   match Task_local_storage.get k_current_fiber with
   | None -> failwith "Fiber.self: must be run from inside a fiber."
   | Some f -> f
 
-let spawn_link_ ?(protect = true) parent f : _ t =
-  (* spawn [f()] with a copy of our local storage *)
-  let ls = Task_local_storage.Private_.Storage.copy_of_current () in
-  let child = spawn_ ~ls:(Some ls) ~on:parent.runner f in
-  add_child_ ~protect parent child;
-  child
-
-let spawn_link ?protect f : _ t =
-  match Task_local_storage.get k_current_fiber with
-  | None -> failwith "Fiber.spawn_link: must be run from inside a fiber."
-  | Some (Any parent) -> spawn_link_ ?protect parent f
-
-let spawn_top_or_link ?protect ~on f : _ t =
-  match Task_local_storage.get_opt k_current_fiber with
-  | Some (Some (Any parent)) -> spawn_link_ ?protect parent f
-  | None | Some None -> spawn_top ~on f
-
-type cancel_handle = int
-
-let add_on_cancel (self : _ t) cb : cancel_handle =
-  let h = ref 0 in
-  while
-    match A.get self.state with
-    | Alive { children; cancel_id; on_cancel } as old ->
-      let new_st =
-        Alive
-          {
-            children;
-            cancel_id = cancel_id + 1;
-            on_cancel = Int_map.add cancel_id cb on_cancel;
-          }
-      in
-      if A.compare_and_set self.state old new_st then (
-        h := cancel_id;
-        false
-      ) else
-        true
-    | Terminating_or_done r ->
-      (match A.get r with
-      | Error ebt -> cb ebt
-      | Ok _ -> ());
-      false
-  do
-    ()
-  done;
-  !h
-
-let remove_on_cancel (self : _ t) h =
-  while
-    match A.get self.state with
-    | Alive ({ on_cancel; _ } as alive) as old ->
-      let new_st =
-        Alive { alive with on_cancel = Int_map.remove h on_cancel }
-      in
-      not (A.compare_and_set self.state old new_st)
-    | Terminating_or_done _ -> false
-  do
-    ()
-  done
-
-let with_cancel_callback (self : _ t) cb (k : unit -> 'a) : 'a =
-  let h = add_on_cancel self cb in
-  Fun.protect k ~finally:(fun () -> remove_on_cancel self h)
+let[@inline] cur_nursery () =
+  let (Any f) = self () in
+  Nursery f
 
 let with_self_cancel_callback cb (k : unit -> 'a) : 'a =
   let (Any self) = self () in
   let h = add_on_cancel self cb in
   Fun.protect k ~finally:(fun () -> remove_on_cancel self h)
 
-let[@inline] await self = Fut.await self.res
-let[@inline] wait_block self = Fut.wait_block self.res
-let[@inline] wait_block_exn self = Fut.wait_block_exn self.res
-
 module Suspend_ = Moonpool.Private.Suspend_
 
 let check_if_cancelled () =
diff --git a/src/fib/fiber.mli b/src/fib/fiber.mli
index a53bbeb3..095ab569 100644
--- a/src/fib/fiber.mli
+++ b/src/fib/fiber.mli
@@ -9,6 +9,46 @@
       of structured concurrency
 *)
 
+type cancel_callback = Exn_bt.t -> unit
+(** A callback used in case of cancellation *)
+
+(** Nurseries.
+
+  Fibers belong in a {b nursery}
+  (as defined in {{: https://trio.readthedocs.io/en/stable/reference-core.html} Trio}).
+
+  The nursery does multiple things.
+
+  - it stores a runner, so we easily know where to run fibers;
+  - it makes it clear in signatures that we might run fibers in a function
+  - it groups cancellation of multiple fibers together
+
+*)
+module Nursery : sig
+  type t
+
+  val await : t -> unit
+  (** Await for the nursery to exit. *)
+
+  val with_create_top : on:Runner.t -> unit -> (t -> 'a) -> 'a
+  (** New toplevel nursery. It runs fibers on the [on] runner. *)
+
+  val with_create_sub : protect:bool -> t -> (t -> 'a) -> 'a
+  (** Create a sub-nursery. The sub-nursery is cancelled if the
+    parent is. Cancelling the sub-nursery also cancels the
+    parent if [protect=false]. When the function returns,
+    the nursery is closed an no other fiber can be scheduled on it. *)
+
+  val cancel_with : t -> Exn_bt.t -> unit
+  (** Cancel the nursery (and all its children) with the given exception. *)
+
+  val with_cancel_callback : t -> cancel_callback -> (unit -> 'a) -> 'a
+  (** [with_cancel_callback nursery cb (fun () -> <e>)] evaluates [e]
+    in a scope in which, if the nursery [nursery] is cancelled,
+    [cb()] is called. If [e] returns without the nursery being cancelled,
+    this callback is removed. *)
+end
+
 (**/**)
 
 module Private_ : sig
@@ -19,6 +59,7 @@ module Private_ : sig
     state: 'a state Atomic.t;  (** Current state in the lifetime of the fiber *)
     res: 'a Fut.t;
     runner: Runner.t;
+    ls: Task_local_storage.storage ref;
   }
   (** Type definition, exposed so that {!any} can be unboxed.
       Please do not rely on that. *)
@@ -39,8 +80,6 @@ val res : 'a t -> 'a Fut.t
 type 'a callback = 'a Exn_bt.result -> unit
 (** Callbacks that are called when a fiber is done. *)
 
-type cancel_callback = Exn_bt.t -> unit
-
 (** Type erased fiber *)
 type any = Private_.any = Any : _ t -> any [@@unboxed]
 
@@ -49,6 +88,11 @@ val self : unit -> any
     Must be run from inside a fiber.
     @raise Failure if not run from inside a fiber. *)
 
+val cur_nursery : unit -> Nursery.t
+(** [cur_nursery ()] returns the nearest nursery.
+    Must be run from inside a fiber.
+    @raise Failure if not run from inside a fiber. *)
+
 val peek : 'a t -> 'a Fut.or_error option
 (** Peek inside the future result *)
 
@@ -107,28 +151,22 @@ val on_result : 'a t -> 'a callback -> unit
     with the result. If the fiber is done already then the
     callback is invoked immediately with its result. *)
 
-val spawn_top : on:Runner.t -> (unit -> 'a) -> 'a t
-(** [spawn_top ~on f] spawns a new (toplevel) fiber onto the given runner.
-    This fiber is not the child of any other fiber: its lifetime
-    is only determined by the lifetime of [f()]. *)
+val spawn : Nursery.t -> ?protect:bool -> (Nursery.t -> 'a) -> 'a t
+(** [spawn n f] spawns a new fiber [fib] in the given nursery [n].
 
-val spawn_link : ?protect:bool -> (unit -> 'a) -> 'a t
-(** [spawn_link ~protect f] spawns a sub-fiber [f_child]
-    from a running fiber [parent].
-    The sub-fiber [f_child] is attached to the current fiber and fails
-    if the current fiber [parent] fails.
+    The fiber [fib] is attached to the nursery and fails
+    if the nursery fails.
 
-    @param protect if true, when [f_child] fails, it does not
-      affect [parent]. If false, [f_child] failing also
-      causes [parent] to fail (and therefore all other children
-      of [parent]). Default is [true].
+    The function [f] is passed a nursery whose lifetime is
+    the same as the fiber's.
 
-    Must be run from inside a fiber.
-    @raise Failure if not run from  inside a fiber. *)
+    @param protect if true, when [fib] fails, it does not
+    affect [nursery] (but the failure can still be re-raised
+    in {!await}). If false, [fib] failing also
+    causes [nursery] to fail (and therefore all other children
+    of [nursery] to be cancelled). Default is [true]. *)
 
-val spawn_top_or_link : ?protect:bool -> on:Runner.t -> (unit -> 'a) -> 'a t
-(** [spawn_top_or_link ~on ~protect f] runs [f()] in a new fiber.
-    If this is run from inside a fiber, this behaves like [spawn_link ~protect f]
-    (links to the parent); otherwise it behaves like [spawn_top ~on f].
-    @param protect if false, failure of the new fiber will also cancel
-    the parent (in case there is a parent). Default [true]. See {!spawn_link}. *)
+val spawn_ignore : Nursery.t -> ?protect:bool -> (Nursery.t -> _) -> unit
+(** [spawn_ignore n f] is [ignore (spawn n f)].
+  The fiber will still affect termination of [n], ie. [n] will exit
+  only after the fiber exits. *)
diff --git a/src/fib/handle.mli b/src/fib/handle.mli
index 1fc5b106..b31e61e4 100644
--- a/src/fib/handle.mli
+++ b/src/fib/handle.mli
@@ -1,4 +1,7 @@
-(** The unique name of a fiber *)
+(** The unique name of a fiber.
+
+    Each fiber has a unique handle that can be used to
+    refer to it in maps or sets. *)
 
 type t = private int
 (** Unique, opaque identifier for a fiber. *)
diff --git a/src/lwt/base.ml b/src/lwt/base.ml
index 88e7ed3d..3b646b05 100644
--- a/src/lwt/base.ml
+++ b/src/lwt/base.ml
@@ -144,13 +144,14 @@ let detach_in_runner ~runner f : _ Lwt.t =
         Perform_action_in_lwt.schedule @@ Action.Wakeup_exn (promise, exn));
   fut
 
-let main_with_runner ~runner (f : unit -> 'a) : 'a =
+let main_with_runner ~runner (f : Fiber.Nursery.t -> 'a) : 'a =
   let lwt_fut, lwt_prom = Lwt.wait () in
+  let@ n = Fiber.Nursery.with_create_top ~on:runner () in
 
   let _fiber =
-    Fiber.spawn_top ~on:runner (fun () ->
+    Fiber.spawn n (fun _n ->
         try
-          let x = f () in
+          let x = f n in
           Perform_action_in_lwt.schedule (Action.Wakeup (lwt_prom, x))
         with exn ->
           Perform_action_in_lwt.schedule (Action.Wakeup_exn (lwt_prom, exn)))
diff --git a/src/lwt/moonpool_lwt.mli b/src/lwt/moonpool_lwt.mli
index ac218e0c..bf7492a0 100644
--- a/src/lwt/moonpool_lwt.mli
+++ b/src/lwt/moonpool_lwt.mli
@@ -136,9 +136,9 @@ val detach_in_runner : runner:Moonpool.Runner.t -> (unit -> 'a) -> 'a Lwt.t
 
 (** {2 Wrappers around Lwt_main} *)
 
-val main_with_runner : runner:Moonpool.Runner.t -> (unit -> 'a) -> 'a
+val main_with_runner : runner:Moonpool.Runner.t -> (Fiber.Nursery.t -> 'a) -> 'a
 (** [main_with_runner ~runner f] starts a Lwt-based event loop and runs [f()] inside
     a fiber in [runner]. *)
 
-val main : (unit -> 'a) -> 'a
+val main : (Fiber.Nursery.t -> 'a) -> 'a
 (** Like {!main_with_runner} but with a default choice of runner. *)