diff --git a/src/prometheus/common_.ml b/src/prometheus/common_.ml
new file mode 100644
index 00000000..bb70b2d7
--- /dev/null
+++ b/src/prometheus/common_.ml
@@ -0,0 +1,3 @@
+module A = Tiny_httpd_atomic_
+
+let spf = Printf.sprintf
diff --git a/src/prometheus/dune b/src/prometheus/dune
new file mode 100644
index 00000000..9cf17df0
--- /dev/null
+++ b/src/prometheus/dune
@@ -0,0 +1,7 @@
+
+(library
+  (name tiny_httpd_prometheus)
+  (public_name tiny_httpd.prometheus)
+  (synopsis "Metrics using prometheus")
+  (private_modules common_)
+  (libraries tiny_httpd))
diff --git a/src/prometheus/tiny_httpd_prometheus.ml b/src/prometheus/tiny_httpd_prometheus.ml
new file mode 100644
index 00000000..026af9db
--- /dev/null
+++ b/src/prometheus/tiny_httpd_prometheus.ml
@@ -0,0 +1,88 @@
+(*
+  https://prometheus.io/docs/instrumenting/exposition_formats/#text-based-format
+  *)
+
+open Common_
+
+let bpf = Printf.bprintf
+
+type tags = (string * string) list
+type counter = { name: string; tags: tags; descr: string option; c: int A.t }
+type gauge = { name: string; tags: tags; descr: string option; g: int A.t }
+type registry = { mutable counters: counter list; mutable gauges: gauge list }
+
+let validate_descr_ what s =
+  if String.contains s '\n' then
+    invalid_arg (spf "%s: description cannot contain '\n'" what)
+
+let emit_tags_ buf tags =
+  if tags <> [] then (
+    bpf buf "{";
+    List.iteri
+      (fun i (k, v) ->
+        if i > 0 then bpf buf ",";
+        bpf buf "%s=%S" k v)
+      tags;
+    bpf buf "}"
+  )
+
+module Counter = struct
+  type t = counter
+
+  let create (reg : registry) ?(tags = []) ?descr name : t =
+    let self : t = { name; descr; tags; c = A.make 0 } in
+    Option.iter (validate_descr_ "counter") descr;
+    reg.counters <- self :: reg.counters;
+    self
+
+  let emit buf (self : t) =
+    Option.iter (bpf buf "# HELP %s %s\n" self.name) self.descr;
+    bpf buf "# TYPE %s counter\n" self.name;
+    bpf buf "%s%a %d\n" self.name emit_tags_ self.tags (A.get self.c);
+    ()
+
+  let[@inline] inc self = A.incr self.c
+  let[@inline] inc_by self n = ignore (A.fetch_and_add self.c n : int)
+  let[@inline] dec self = A.decr self.c
+  let[@inline] dec_by self n = ignore (A.fetch_and_add self.c (-n) : int)
+end
+
+module Gauge = struct
+  type t = gauge
+
+  let create (reg : registry) ?(tags = []) ?descr name : t =
+    Option.iter (validate_descr_ "gauge") descr;
+    let self : t = { name; descr; tags; g = A.make 0 } in
+    reg.gauges <- self :: reg.gauges;
+    self
+
+  let emit buf (self : t) =
+    Option.iter (bpf buf "# HELP %s %s\n" self.name) self.descr;
+    bpf buf "# TYPE %s gauge\n" self.name;
+    bpf buf "%s%a %d\n" self.name emit_tags_ self.tags (A.get self.g);
+    ()
+
+  let[@inline] set self x = A.set self.g x
+  let[@inline] inc self = A.incr self.g
+  let[@inline] inc_by self n = ignore (A.fetch_and_add self.g n : int)
+  let[@inline] dec self = A.decr self.g
+  let[@inline] dec_by self n = ignore (A.fetch_and_add self.g (-n) : int)
+end
+
+module Registry = struct
+  type t = registry
+
+  let create () : t = { counters = []; gauges = [] }
+
+  let emit (buf : Buffer.t) (self : t) : unit =
+    List.iter (Gauge.emit buf) self.gauges;
+    List.iter (Counter.emit buf) self.counters;
+    ()
+
+  let emit_str (self : t) : string =
+    let buf = Buffer.create 32 in
+    emit buf self;
+    Buffer.contents buf
+end
+
+let global = Registry.create ()
diff --git a/src/prometheus/tiny_httpd_prometheus.mli b/src/prometheus/tiny_httpd_prometheus.mli
new file mode 100644
index 00000000..0cd603f8
--- /dev/null
+++ b/src/prometheus/tiny_httpd_prometheus.mli
@@ -0,0 +1,49 @@
+(** Expose metrics over HTTP in the prometheus format *)
+
+type tags = (string * string) list
+
+(** Registry for metrics. *)
+module Registry : sig
+  type t
+  (** The registry contains a group of metrics *)
+
+  val create : unit -> t
+
+  val emit : Buffer.t -> t -> unit
+  (** Write metrics into the given buffer. The buffer will be
+      cleared first thing. *)
+
+  val emit_str : t -> string
+end
+
+val global : Registry.t
+
+(** Counters *)
+module Counter : sig
+  type t
+  (** A counter, monotonically increasing *)
+
+  val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
+  val inc : t -> unit
+  val inc_by : t -> int -> unit
+  val dec : t -> unit
+  val dec_by : t -> int -> unit
+end
+
+(** Gauges *)
+module Gauge : sig
+  type t
+  (** A gauge, taking arbitrary values *)
+
+  val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
+  val set : t -> int -> unit
+  val inc : t -> unit
+  val inc_by : t -> int -> unit
+  val dec : t -> unit
+  val dec_by : t -> int -> unit
+end
+
+(* TODO:
+   module Histogram : sig
+   end
+*)