diff --git a/README.md b/README.md
index c03a7644..e7bd23ee 100644
--- a/README.md
+++ b/README.md
@@ -137,6 +137,7 @@ Documentation [here](http://cedeela.fr/~simon/software/containers).
 - `CCHashconsedSet`, a set structure with sharing of sub-structures
 - `CCGraph`, a small collection of graph algorithms
 - `CCBitField`, a type-safe implementation of bitfields that fit in `int`
+- `CCWBTree`, a weight-balanced tree, implementing a map interface
 
 ### Containers.io
 
diff --git a/_oasis b/_oasis
index c50b3f31..2847d801 100644
--- a/_oasis
+++ b/_oasis
@@ -85,7 +85,7 @@ Library "containers_data"
                     CCPersistentHashtbl, CCDeque, CCFQueue, CCBV, CCMixtbl,
                     CCMixmap, CCRingBuffer, CCIntMap, CCPersistentArray,
                     CCMixset, CCHashconsedSet, CCGraph, CCHashSet, CCBitField,
-                    CCHashTrie, CCBloom
+                    CCHashTrie, CCBloom, CCWBTree
   BuildDepends:     bytes
   FindlibParent:    containers
   FindlibName:      data
diff --git a/benchs/run_benchs.ml b/benchs/run_benchs.ml
index 52795f40..e10646d2 100644
--- a/benchs/run_benchs.ml
+++ b/benchs/run_benchs.ml
@@ -287,6 +287,17 @@ module Tbl = struct
     let module U = MUT_OF_IMMUT(T) in
     (module U : MUT with type key = a)
 
+  let wbt : type a. a key_type -> (module MUT with type key = a)
+  = fun k ->
+    let (module K), name = arg_make k in
+    let module T = struct
+      let name = sprintf "wbt(%s)" name
+      include CCWBTree.Make(K)
+      let find = get_exn
+    end in
+    let module U = MUT_OF_IMMUT(T) in
+    (module U : MUT with type key = a)
+
   let flat_hashtbl =
     let module T = CCFlatHashtbl.Make(CCInt) in
     let module U = struct
@@ -328,6 +339,7 @@ module Tbl = struct
     ; persistent_hashtbl
     ; poly_hashtbl
     ; map Int
+    ; wbt Int
     ; flat_hashtbl
     ; hashtrie Int
     ; hamt Int
@@ -336,6 +348,7 @@ module Tbl = struct
   let modules_string =
     [ hashtbl_make Str
     ; map Str
+    ; wbt Str
     ; hashtrie Str
     ; hamt Str
     ]
diff --git a/doc/intro.txt b/doc/intro.txt
index 16b9db22..0cfd1dbf 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -81,6 +81,7 @@ CCPersistentArray
 CCPersistentHashtbl
 CCRingBuffer
 CCTrie
+CCWBTree
 }
 
 {4 Containers.io}
diff --git a/src/data/CCWBTree.ml b/src/data/CCWBTree.ml
new file mode 100644
index 00000000..fe4cd5f5
--- /dev/null
+++ b/src/data/CCWBTree.ml
@@ -0,0 +1,332 @@
+(* This file is free software, part of containers. See file "license" for more details. *)
+
+(** {1 Weight-Balanced Tree} *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
+type 'a printer = Format.formatter -> 'a -> unit
+
+module type ORD = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module type KEY = sig
+  include ORD
+  val weight : t -> int
+end
+
+(** {2 Signature} *)
+
+module type S = sig
+  type key
+
+  type 'a t
+
+  val empty : 'a t
+
+  val mem : key -> _ t -> bool
+
+  val get : key -> 'a t -> 'a option
+
+  val get_exn : key -> 'a t -> 'a
+  (** @raise Not_found if the key is not present *)
+
+  val nth : int -> 'a t -> (key * 'a) option
+  (** [nth i m] returns the [i]-th [key, value] in the ascending
+      order. Complexity is [O(log (cardinal m))] *)
+
+  val nth_exn : int -> 'a t -> key * 'a
+  (** @raise Not_found if the index is invalid *)
+
+  val add : key -> 'a -> 'a t -> 'a t
+
+  val remove : key -> 'a t -> 'a t
+
+  val cardinal : _ t -> int
+
+  val weight : _ t -> int
+
+  val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
+
+  val iter : (key -> 'a -> unit) -> 'a t -> unit
+
+  val choose : 'a t -> (key * 'a) option
+
+  val choose_exn : 'a t -> key * 'a
+  (** @raise Not_found if the tree is empty *)
+
+  val random_choose : Random.State.t -> 'a t -> key * 'a
+  (** Randomly choose a (key,value) pair within the tree, using weights
+      as probability weights
+      @raise Not_found if the tree is empty *)
+
+  val add_list : 'a t -> (key * 'a) list -> 'a t
+
+  val of_list : (key * 'a) list -> 'a t
+
+  val to_list : 'a t -> (key * 'a) list
+
+  val add_seq : 'a t -> (key * 'a) sequence -> 'a t
+
+  val of_seq : (key * 'a) sequence -> 'a t
+
+  val to_seq : 'a t -> (key * 'a) sequence
+
+  val add_gen : 'a t -> (key * 'a) gen -> 'a t
+
+  val of_gen : (key * 'a) gen -> 'a t
+
+  val to_gen : 'a t -> (key * 'a) gen
+
+  val print : key printer -> 'a printer -> 'a t printer
+
+  (**/**)
+  val balanced : _ t -> bool
+  (**/**)
+end
+
+module MakeFull(K : KEY) : S with type key = K.t = struct
+  type key = K.t
+
+  type weight = int
+
+  type 'a t =
+    | E
+    | N of key * 'a * 'a t * 'a t * weight
+
+  let empty = E
+
+  let rec get_exn k m = match m with
+    | E -> raise Not_found
+    | N (k', v, l, r, _) ->
+        match K.compare k k' with
+        | 0 -> v
+        | n when n<0 -> get_exn k l
+        | _ -> get_exn k r
+
+  let get k m =
+    try Some (get_exn k m)
+    with Not_found -> None
+
+  let mem k m =
+    try ignore (get_exn k m); true
+    with Not_found -> false
+
+  let singleton k v =
+    N (k, v, E, E, K.weight k)
+
+  let weight = function
+    | E -> 0
+    | N (_, _, _, _, w) -> w
+
+  (* balancing parameters *)
+
+  (* delta=5/2
+     delta × (weight l + 1) ≥ weight r + 1
+  *)
+  let is_balanced l r =
+    5 * (weight l + 1) >= (weight r + 1) * 2
+
+  (* gamma = 3/2
+      weight l + 1 < gamma × (weight r + 1) *)
+  let is_single l r =
+    2 * (weight l + 1) < 3 * (weight r + 1)
+
+  (* debug function *)
+  let rec balanced = function
+    | E -> true
+    | N (_, _, l, r, _) ->
+        is_balanced l r &&
+        is_balanced r l &&
+        balanced l &&
+        balanced r
+
+  (* smart constructor *)
+  let mk_node_ k v l r =
+    N (k, v, l, r, weight l + weight r + K.weight k)
+
+  let single_l k1 v1 t1 t2 = match t2 with
+    | E -> assert false
+    | N (k2, v2, t2, t3, _) ->
+        mk_node_ k2 v2 (mk_node_ k1 v1 t1 t2) t3
+
+  let double_l k1 v1 t1 t2 = match t2 with
+    | N (k2, v2, N (k3, v3, t2, t3, _), t4, _) ->
+        mk_node_ k3 v3 (mk_node_ k1 v1 t1 t2) (mk_node_ k2 v2 t3 t4)
+    | _ -> assert false
+
+  let rotate_l k v l r = match r with
+    | E -> assert false
+    | N (_, _, rl, rr, _) ->
+        if is_single rl rr
+        then single_l k v l r
+        else double_l k v l r
+
+  (* balance towards left *)
+  let balance_l k v l r =
+    if is_balanced l r then mk_node_ k v l r
+    else rotate_l k v l r
+
+  let single_r k1 v1 t1 t2 = match t1 with
+    | E -> assert false
+    | N (k2, v2, t11, t12, _) ->
+        mk_node_ k2 v2 t11 (mk_node_ k1 v1 t12 t2)
+
+  let double_r k1 v1 t1 t2 = match t1 with
+    | N (k2, v2, t11, N (k3, v3, t121, t122, _), _) ->
+        mk_node_ k3 v3 (mk_node_ k2 v2 t11 t121) (mk_node_ k1 v1 t122 t2)
+    | _ -> assert false
+
+  let rotate_r k v l r = match l with
+    | E -> assert false
+    | N (_, _, ll, lr, _) ->
+        if is_single lr ll
+        then single_r k v l r
+        else double_r k v l r
+
+  (* balance toward right *)
+  let balance_r k v l r =
+    if is_balanced r l then mk_node_ k v l r
+    else rotate_r k v l r
+
+  let rec add k v m = match m with
+    | E -> singleton k v
+    | N (k', v', l, r, _) ->
+        match K.compare k k' with
+        | 0 -> mk_node_ k v l r
+        | n when n<0 -> balance_r k' v' (add k v l) r
+        | _ -> balance_l k' v' l (add k v r)
+
+  (*$Q & ~small:List.length
+    Q.(list (pair small_int bool)) (fun l -> \
+      let module M = Make(CCInt) in \
+      let m = M.of_list l in \
+      M.balanced m)
+    Q.(list (pair small_int small_int)) (fun l -> \
+      let l = CCList.Set.uniq ~eq:(CCFun.compose_binop fst (=)) l in \
+      let module M = Make(CCInt) in \
+      let m = M.of_list l in \
+      List.for_all (fun (k,v) -> M.get_exn k m = v) l)
+    Q.(list (pair small_int small_int)) (fun l -> \
+      let l = CCList.Set.uniq ~eq:(CCFun.compose_binop fst (=)) l in \
+      let module M = Make(CCInt) in \
+      let m = M.of_list l in \
+      M.cardinal m = List.length l)
+  *)
+
+  let rec remove k m = match m with
+    | E -> E
+    | N (k', v', l, r, _) ->
+        match K.compare k k' with
+        | 0 -> assert false (* TODO fix using a paper *)
+        | n when n<0 -> balance_l k' v' (remove k l) r
+        | _ -> balance_r k' v' l (remove k r)
+
+  (* TODO union, intersection *)
+
+  let rec nth_exn i m = match m with
+    | E -> raise Not_found
+    | N (k, v, l, r, w) ->
+        let c = i - weight l in
+        match c with
+        | 0 -> k, v
+        | n when n<0 -> nth_exn i l  (* search left *)
+        | _ ->
+            (* means c< K.weight k *)
+            if i<w-weight r then k,v else nth_exn (i+weight r-w) r
+
+  let nth i m =
+    try Some (nth_exn i m)
+    with Not_found -> None
+
+  (*$T
+    let module M = Make(CCInt) in \
+    let m = CCList.(0 -- 1000 |> map (fun i->i,i) |> M.of_list) in \
+    List.for_all (fun i -> M.nth_exn i m = (i,i)) CCList.(0--1000)
+  *)
+
+  let rec fold f acc m = match m with
+    | E -> acc
+    | N (k, v, l, r, _) ->
+        let acc = fold f acc l in
+        let acc = f acc k v in
+        fold f acc r
+
+  let rec iter f m = match m with
+    | E -> ()
+    | N (k, v, l, r, _) ->
+        iter f l;
+        f k v;
+        iter f r
+
+  let choose_exn = function
+    | E -> raise Not_found
+    | N (k, v, _, _, _) -> k, v
+
+  let choose = function
+    | E -> None
+    | N (k, v, _, _, _) -> Some (k,v)
+
+  (* pick an index within [0.. weight m-1] and get the element with
+     this index *)
+  let random_choose st m =
+    let w = weight m in
+    if w=0 then raise Not_found;
+    nth_exn (Random.State.int st w) m
+
+  let cardinal m = fold (fun acc _ _ -> acc+1) 0 m
+
+  let add_list m l = List.fold_left (fun acc (k,v) -> add k v acc) m l
+
+  let of_list l = add_list empty l
+
+  let to_list m = fold (fun acc k v -> (k,v) :: acc) [] m
+
+  let add_seq m seq =
+    let m = ref m in
+    seq (fun (k,v) -> m := add k v !m);
+    !m
+
+  let of_seq s = add_seq empty s
+
+  let to_seq m yield = iter (fun k v -> yield (k,v)) m
+
+  let rec add_gen m g = match g() with
+    | None -> m
+    | Some (k,v) -> add_gen (add k v m) g
+
+  let of_gen g = add_gen empty g
+
+  let to_gen m =
+    let st = Stack.create () in
+    Stack.push m st;
+    let rec next() =
+      if Stack.is_empty st then None
+      else match Stack.pop st with
+        | E -> next ()
+        | N (k, v, l, r, _) ->
+            Stack.push r st;
+            Stack.push l st;
+            Some (k,v)
+    in next
+
+  let print pp_k pp_v fmt m =
+    let start = "[" and stop = "]" and arrow = "->" and sep = ","in
+    Format.pp_print_string fmt start;
+    let first = ref true in
+    iter
+      (fun k v ->
+        if !first then first := false else Format.pp_print_string fmt sep;
+        pp_k fmt k;
+        Format.pp_print_string fmt arrow;
+        pp_v fmt v;
+        Format.pp_print_cut fmt ()
+      ) m;
+    Format.pp_print_string fmt stop
+end
+
+module Make(X : ORD) = MakeFull(struct
+  include X
+  let weight _ = 1
+end)
diff --git a/src/data/CCWBTree.mli b/src/data/CCWBTree.mli
new file mode 100644
index 00000000..87eb975e
--- /dev/null
+++ b/src/data/CCWBTree.mli
@@ -0,0 +1,99 @@
+
+(* This file is free software, part of containers. See file "license" for more details. *)
+
+(** {1 Weight-Balanced Tree}
+
+  {b status: experimental}
+
+  @since NEXT_RELEASE *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
+type 'a printer = Format.formatter -> 'a -> unit
+
+module type ORD = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module type KEY = sig
+  include ORD
+  val weight : t -> int
+end
+
+(** {2 Signature} *)
+
+module type S = sig
+  type key
+
+  type 'a t
+
+  val empty : 'a t
+
+  val mem : key -> _ t -> bool
+
+  val get : key -> 'a t -> 'a option
+
+  val get_exn : key -> 'a t -> 'a
+  (** @raise Not_found if the key is not present *)
+
+  val nth : int -> 'a t -> (key * 'a) option
+  (** [nth i m] returns the [i]-th [key, value] in the ascending
+      order. Complexity is [O(log (cardinal m))] *)
+
+  val nth_exn : int -> 'a t -> key * 'a
+  (** @raise Not_found if the index is invalid *)
+
+  val add : key -> 'a -> 'a t -> 'a t
+
+  val remove : key -> 'a t -> 'a t
+
+  val cardinal : _ t -> int
+
+  val weight : _ t -> int
+
+  val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
+
+  val iter : (key -> 'a -> unit) -> 'a t -> unit
+
+  val choose : 'a t -> (key * 'a) option
+
+  val choose_exn : 'a t -> key * 'a
+  (** @raise Not_found if the tree is empty *)
+
+  val random_choose : Random.State.t -> 'a t -> key * 'a
+  (** Randomly choose a (key,value) pair within the tree, using weights
+      as probability weights
+      @raise Not_found if the tree is empty *)
+
+  val add_list : 'a t -> (key * 'a) list -> 'a t
+
+  val of_list : (key * 'a) list -> 'a t
+
+  val to_list : 'a t -> (key * 'a) list
+
+  val add_seq : 'a t -> (key * 'a) sequence -> 'a t
+
+  val of_seq : (key * 'a) sequence -> 'a t
+
+  val to_seq : 'a t -> (key * 'a) sequence
+
+  val add_gen : 'a t -> (key * 'a) gen -> 'a t
+
+  val of_gen : (key * 'a) gen -> 'a t
+
+  val to_gen : 'a t -> (key * 'a) gen
+
+  val print : key printer -> 'a printer -> 'a t printer
+
+  (**/**)
+  val balanced : _ t -> bool
+  (**/**)
+end
+
+(** {2 Functor} *)
+
+module Make(X : ORD) : S with type key = X.t
+
+module MakeFull(X : KEY) : S with type key = X.t
+(** Use the custom [X.weight] function *)