diff --git a/src/action.ml b/src/action.ml
index 14dc238a..f274750b 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -45,6 +45,7 @@ module Mini_shexp = struct
       | Copy_and_add_line_directive of 'path * 'path
       | System         of 'a
       | Bash           of 'a
+      | Write_file     of 'path * 'a
 
     let rec t a p sexp =
       sum
@@ -85,6 +86,7 @@ module Mini_shexp = struct
         List [Atom "copy-and-add-line-directive"; g x; g y]
       | System x -> List [Atom "system"; f x]
       | Bash   x -> List [Atom "bash"; f x]
+      | Write_file (x, y) -> List [Atom "write-file"; g x; f y]
 
     let rec fold t ~init:acc ~f =
       match t with
@@ -101,6 +103,7 @@ module Mini_shexp = struct
       | Copy_and_add_line_directive (x, y) -> f (f acc x) y
       | System x -> f acc x
       | Bash x -> f acc x
+      | Write_file (x, y) -> f (f acc x) y
   end
   open Ast
 
@@ -148,6 +151,7 @@ module Mini_shexp = struct
         Copy_and_add_line_directive (expand_path ~dir ~f x, expand_path ~dir ~f y)
       | System x -> System (expand_str ~dir ~f x)
       | Bash x -> Bash (expand_str ~dir ~f x)
+      | Write_file (x, y) -> Write_file (expand_path ~dir ~f x, expand_str ~dir ~f y)
   end
 
   open Future
@@ -246,6 +250,13 @@ module Mini_shexp = struct
       run ~dir ~env ~env_extra ~stdout_to ~tail
         (Path.absolute "/bin/bash")
         ["-e"; "-u"; "-o"; "pipefail"; "-c"; cmd]
+    | Write_file (fn, s) ->
+      let fn = Path.to_string fn in
+      if Sys.file_exists fn && read_file fn = s then
+        ()
+      else
+        write_file fn s;
+      return ()
 
   and exec_list l ~dir ~env ~env_extra ~stdout_to ~tail =
     match l with
@@ -301,3 +312,10 @@ let exec { action; dir; context } =
   in
   Mini_shexp.exec action ~dir ~env ~env_extra:String_map.empty
     ~stdout_to:None ~tail:true
+
+type for_hash = string option * Path.t * Mini_shexp.t
+
+let for_hash { context; dir; action } =
+  (Option.map context ~f:(fun c -> c.name),
+   dir,
+   action)
diff --git a/src/action.mli b/src/action.mli
index 5f75c6e2..9184a422 100644
--- a/src/action.mli
+++ b/src/action.mli
@@ -22,6 +22,7 @@ module Mini_shexp : sig
       | Copy_and_add_line_directive of 'path * 'path
       | System         of 'a
       | Bash           of 'a
+      | Write_file     of 'path * 'a
     val t : 'a Sexp.Of_sexp.t -> 'b Sexp.Of_sexp.t -> ('a, 'b) t Sexp.Of_sexp.t
     val sexp_of_t : 'a Sexp.To_sexp.t -> 'b Sexp.To_sexp.t -> ('a, 'b) t Sexp.To_sexp.t
   end
@@ -49,3 +50,6 @@ type t =
 val t : Context.t String_map.t -> t Sexp.Of_sexp.t
 val sexp_of_t : t Sexp.To_sexp.t
 val exec : t -> unit Future.t
+
+type for_hash
+val for_hash : t -> for_hash
diff --git a/src/build.ml b/src/build.ml
index 27fdf6a2..024be6da 100644
--- a/src/build.ml
+++ b/src/build.ml
@@ -163,7 +163,7 @@ let action ?(dir=Path.root) ?context ~targets action =
   { Action. context; dir; action }
 
 let echo fn s =
-  action ~targets:[fn] (With_stdout_to (fn, Echo s))
+  action ~targets:[fn] (Write_file (fn, s))
 
 let echo_dyn fn =
   Targets [fn]
@@ -171,7 +171,7 @@ let echo_dyn fn =
   { Action.
     context = None
   ; dir     = Path.root
-  ; action  = With_stdout_to (fn, Echo s)
+  ; action  = Write_file (fn, s)
   }
 
 let copy ~src ~dst =
diff --git a/src/build_system.ml b/src/build_system.ml
index 3a944266..27c1c6d7 100644
--- a/src/build_system.ml
+++ b/src/build_system.ml
@@ -20,10 +20,10 @@ end
 
 module Rule = struct
   type t =
-    { deps         : Pset.t
-    ; targets      : Pset.t
-    ; build        : (unit, Action.t) Build.t
-    ; mutable exec : Exec_status.t
+    { deps           : Pset.t
+    ; targets        : Pset.t
+    ; build          : (unit, Action.t) Build.t
+    ; mutable exec   : Exec_status.t
     }
 end
 
@@ -56,10 +56,32 @@ end
 
 type t =
   { (* File specification by targets *)
-    files    : (Path.t, File_spec.packed) Hashtbl.t
-  ; contexts : Context.t list
+    files      : (Path.t, File_spec.packed) Hashtbl.t
+  ; contexts   : Context.t list
+  ; (* Table from target to digest of [(deps, targets, action)] *)
+    trace      : (Path.t, Digest.t) Hashtbl.t
+  ; timestamps : (Path.t, float) Hashtbl.t
   }
 
+let timestamp t fn ~default =
+  match Hashtbl.find t.timestamps fn with
+  | Some ts -> ts
+  | None ->
+    match Unix.lstat (Path.to_string fn) with
+    | exception _ -> default
+    | stat        ->
+      let ts = stat.st_mtime in
+      Hashtbl.add t.timestamps ~key:fn ~data:ts;
+      ts
+
+let min_timestamp t fns =
+  List.fold_left fns ~init:max_float
+    ~f:(fun acc fn -> min acc (timestamp t fn ~default:0.))
+
+let max_timestamp t fns =
+  List.fold_left fns ~init:0.
+    ~f:(fun acc fn -> max acc (timestamp t fn ~default:max_float))
+
 let find_file_exn t file =
   Hashtbl.find_exn t.files file ~string_of_key:(fun fn -> sprintf "%S" (Path.to_string fn))
     ~table_desc:(fun _ -> "<target to rule>")
@@ -146,20 +168,14 @@ let get_file : type a. t -> Path.t -> a File_kind.t -> a File_spec.t = fun t fn
     let Eq = File_kind.eq_exn kind file.kind in
     file
 
-let save_vfile (type a) (module K : Vfile_kind.S with type t = a) fn x =
-  K.save fn x
+let vfile_to_string (type a) (module K : Vfile_kind.S with type t = a) fn x =
+  K.to_string fn x
 
 module Build_exec = struct
   open Build.Repr
 
-  let nop =
-    { Action.
-      context = None
-    ; dir     = Path.root
-    ; action  = Progn []
-    }
-
-  let exec bs t x ~targeting =
+  let exec bs t x ~static_deps ~targeting =
+    let all_deps = ref static_deps in
     let rec exec
       : type a b. (a, b) t -> a -> b Future.t = fun t x ->
       let return = Future.return in
@@ -170,8 +186,12 @@ module Build_exec = struct
         let file = get_file bs fn (Sexp_file kind) in
         assert (file.data = None);
         file.data <- Some x;
-        save_vfile kind fn x;
-        Future.return nop
+        Future.return
+          { Action.
+            context = None
+          ; dir     = Path.root
+          ; action  = Write_file (fn, vfile_to_string kind fn x)
+          }
       | Compose (a, b) ->
         exec a x >>= exec b
       | First t ->
@@ -194,12 +214,14 @@ module Build_exec = struct
         return (Option.value_exn file.data)
       | Dyn_paths t ->
         exec t x >>= fun fns ->
+        all_deps := Pset.union !all_deps (Pset.of_list fns);
         all_unit (List.rev_map fns ~f:(wait_for_file bs ~targeting)) >>= fun () ->
         return x
       | Record_lib_deps _ -> return x
       | Fail { fail } -> fail ()
     in
-    exec (Build.repr t) x
+    exec (Build.repr t) x >>| fun action ->
+    (action, !all_deps)
 end
 
 let add_spec t fn spec ~allow_override =
@@ -252,12 +274,31 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
     all_unit
       (Pset.fold deps ~init:[] ~f:(fun fn acc -> wait_for_file t fn ~targeting :: acc))
     >>= fun () ->
-    Build_exec.exec t build () ~targeting
-    >>= fun action ->
+    Build_exec.exec t build () ~targeting ~static_deps:deps
+    >>= fun (action, all_deps) ->
     if !Clflags.debug_actions then
       Format.eprintf "@{<debug>Action@}: %s@."
         (Sexp.to_string (Action.sexp_of_t action));
-    Action.exec action
+    let all_deps = Pset.elements all_deps in
+    let targets  = Pset.elements targets  in
+    let hash =
+      let trace = (all_deps, targets, Action.for_hash action) in
+      Digest.string (Marshal.to_string trace [])
+    in
+    let rule_changed =
+      List.fold_left targets ~init:false ~f:(fun acc fn ->
+        match Hashtbl.find t.trace fn with
+        | None ->
+          Hashtbl.add t.trace ~key:fn ~data:hash;
+          true
+        | Some prev_hash ->
+          Hashtbl.replace t.trace ~key:fn ~data:hash;
+          acc || prev_hash <> hash)
+    in
+    if rule_changed || min_timestamp t targets < max_timestamp t all_deps then
+      Action.exec action
+    else
+      return ()
   ) in
   let rule =
     { Rule.
@@ -290,6 +331,36 @@ let setup_copy_rules t ~all_non_target_source_files ~all_targets_by_dir =
           ~all_targets_by_dir
           ~allow_override:true))
 
+module Trace = struct
+  type t = (Path.t, Digest.t) Hashtbl.t
+
+  let file = "_build/.db"
+
+  let dump (trace : t) =
+    let sexp =
+      Sexp.List (
+        Hashtbl.fold trace ~init:Pmap.empty ~f:(fun ~key ~data acc ->
+          Pmap.add acc ~key ~data)
+        |> Path.Map.bindings
+        |> List.map ~f:(fun (path, hash) ->
+          Sexp.List [ Atom (Path.to_string path); Atom (Digest.to_hex hash) ]))
+    in
+    write_file file (Sexp.to_string sexp)
+
+  let load () =
+    let trace = Hashtbl.create 1024 in
+    if Sys.file_exists file then begin
+      let sexp = Sexp_load.single file in
+      let bindings =
+        let open Sexp.Of_sexp in
+        list (pair Path.t (fun s -> Digest.from_hex (string s))) sexp
+      in
+      List.iter bindings ~f:(fun (path, hash) ->
+        Hashtbl.add trace ~key:path ~data:hash);
+    end;
+    trace
+end
+
 let create ~contexts ~file_tree ~rules =
   let all_source_files =
     File_tree.fold file_tree ~init:Pset.empty ~f:(fun dir acc ->
@@ -321,11 +392,17 @@ let create ~contexts ~file_tree ~rules =
     |> Pmap.of_alist_multi
     |> Pmap.map ~f:Pset.of_list
   ) in
-  let t = { files = Hashtbl.create 1024; contexts } in
+  let t =
+    { contexts
+    ; files      = Hashtbl.create 1024
+    ; trace      = Trace.load ()
+    ; timestamps = Hashtbl.create 1024
+    } in
   List.iter rules ~f:(compile_rule t ~all_targets_by_dir ~allow_override:false);
   setup_copy_rules t ~all_targets_by_dir
     ~all_non_target_source_files:
       (Pset.diff all_source_files all_other_targets);
+  at_exit (fun () -> Trace.dump t.trace);
   t
 
 let remove_old_artifacts t =
diff --git a/src/sexp.ml b/src/sexp.ml
index ab65fdd1..644b4548 100644
--- a/src/sexp.ml
+++ b/src/sexp.ml
@@ -80,6 +80,8 @@ module To_sexp = struct
     | Some x -> List [f x]
   let string_set set = list string (String_set.elements set)
   let string_map f map = list (pair string f) (String_map.bindings map)
+  let record l =
+    List (List.map l ~f:(fun (n, v) -> List [Atom n; v]))
 end
 
 module Of_sexp = struct
diff --git a/src/sexp.mli b/src/sexp.mli
index 17c9f538..973e379d 100644
--- a/src/sexp.mli
+++ b/src/sexp.mli
@@ -36,7 +36,12 @@ module type Combinators = sig
   val string_map : 'a t -> 'a String_map.t   t
 end
 
-module To_sexp : Combinators with type 'a t = 'a -> t
+module To_sexp : sig
+  type sexp = t
+  include Combinators with type 'a t = 'a -> t
+
+  val record : (string * sexp) list -> sexp
+end with type sexp := t
 
 module Of_sexp : sig
   type ast = Ast.t =
diff --git a/src/vfile_kind.ml b/src/vfile_kind.ml
index 1e0b97bc..3a3092c3 100644
--- a/src/vfile_kind.ml
+++ b/src/vfile_kind.ml
@@ -32,7 +32,7 @@ module type S = sig
   val id : t Id.t
 
   val load : Path.t -> t
-  val save : Path.t -> t -> unit
+  val to_string : Path.t -> t -> string
 end
 
 type 'a t = (module S with type t = 'a)
@@ -52,11 +52,7 @@ struct
 
   let id = Id.create ()
 
-  let save path x =
-    let s = To_sexp.t path x |> Sexp.to_string in
-    let oc = open_out (Path.to_string path) in
-    output_string oc s;
-    close_out oc
+  let to_string path x = To_sexp.t path x |> Sexp.to_string
 
   let load path =
     Of_sexp.t path (Sexp_load.single (Path.to_string path))
diff --git a/src/vfile_kind.mli b/src/vfile_kind.mli
index 4ee25ccf..447b910d 100644
--- a/src/vfile_kind.mli
+++ b/src/vfile_kind.mli
@@ -12,7 +12,7 @@ module type S = sig
   val id : t Id.t
 
   val load : Path.t -> t
-  val save : Path.t -> t -> unit
+  val to_string : Path.t -> t -> string
 end
 
 type 'a t = (module S with type t = 'a)