diff --git a/src/event/dune b/src/event/dune
new file mode 100644
index 0000000..051a6f0
--- /dev/null
+++ b/src/event/dune
@@ -0,0 +1,7 @@
+
+(library
+ (name trace_event)
+ (public_name trace.event)
+ (synopsis "Turns subscriber callbacks into an event type")
+ (libraries
+  (re_export trace.core) (re_export trace.subscriber)))
diff --git a/src/event/event.ml b/src/event/event.ml
new file mode 100644
index 0000000..7ccb4d2
--- /dev/null
+++ b/src/event/event.ml
@@ -0,0 +1,63 @@
+open Trace_core
+module Sub = Trace_subscriber
+
+(** An event with TEF/fuchsia semantics *)
+type t =
+  | E_tick
+  | E_init of { time_ns: int64 }
+  | E_shutdown of { time_ns: int64 }
+  | E_message of {
+      tid: int;
+      msg: string;
+      time_ns: int64;
+      data: (string * Sub.user_data) list;
+    }
+  | E_define_span of {
+      tid: int;
+      name: string;
+      time_ns: int64;
+      id: span;
+      fun_name: string option;
+      data: (string * Sub.user_data) list;
+    }
+  | E_exit_span of {
+      id: span;
+      time_ns: int64;
+    }
+  | E_add_data of {
+      id: span;
+      data: (string * Sub.user_data) list;
+    }
+  | E_enter_manual_span of {
+      tid: int;
+      name: string;
+      time_ns: int64;
+      id: trace_id;
+      flavor: Sub.flavor option;
+      fun_name: string option;
+      data: (string * Sub.user_data) list;
+    }
+  | E_exit_manual_span of {
+      tid: int;
+      name: string;
+      time_ns: int64;
+      flavor: Sub.flavor option;
+      data: (string * Sub.user_data) list;
+      id: trace_id;
+    }
+  | E_counter of {
+      name: string;
+      tid: int;
+      time_ns: int64;
+      n: float;
+    }
+  | E_name_process of { name: string }
+  | E_name_thread of {
+      tid: int;
+      name: string;
+    }
+  | E_extension_event of {
+      tid: int;
+      time_ns: int64;
+      ext: Trace_core.extension_event;
+    }
diff --git a/src/event/subscriber.ml b/src/event/subscriber.ml
new file mode 100644
index 0000000..11edab7
--- /dev/null
+++ b/src/event/subscriber.ml
@@ -0,0 +1,53 @@
+open Trace_core
+open Event
+
+type event_consumer = { on_event: Event.t -> unit } [@@unboxed]
+(** Callback for events *)
+
+module Callbacks : Sub.Callbacks.S with type st = event_consumer = struct
+  type st = event_consumer
+
+  let on_init (self : st) ~time_ns = self.on_event (E_init { time_ns })
+  let on_shutdown (self : st) ~time_ns = self.on_event (E_shutdown { time_ns })
+
+  let on_name_process (self : st) ~time_ns:_ ~tid:_ ~name : unit =
+    self.on_event @@ E_name_process { name }
+
+  let on_name_thread (self : st) ~time_ns:_ ~tid ~name : unit =
+    self.on_event @@ E_name_thread { tid; name }
+
+  let[@inline] on_enter_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
+      ~__LINE__:_ ~time_ns ~tid ~data ~name span : unit =
+    self.on_event
+    @@ E_define_span { tid; name; time_ns; id = span; fun_name; data }
+
+  let on_exit_span (self : st) ~time_ns ~tid:_ span : unit =
+    self.on_event @@ E_exit_span { id = span; time_ns }
+
+  let on_add_data (self : st) ~data span =
+    if data <> [] then self.on_event @@ E_add_data { id = span; data }
+
+  let on_message (self : st) ~time_ns ~tid ~span:_ ~data msg : unit =
+    self.on_event @@ E_message { tid; time_ns; msg; data }
+
+  let on_counter (self : st) ~time_ns ~tid ~data:_ ~name f : unit =
+    self.on_event @@ E_counter { name; n = f; time_ns; tid }
+
+  let on_enter_manual_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
+      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor ~trace_id _span :
+      unit =
+    self.on_event
+    @@ E_enter_manual_span
+         { id = trace_id; time_ns; tid; data; name; fun_name; flavor }
+
+  let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor
+      ~trace_id (_ : span) : unit =
+    self.on_event
+    @@ E_exit_manual_span { tid; id = trace_id; name; time_ns; data; flavor }
+
+  let on_extension_event (self : st) ~time_ns ~tid ext : unit =
+    self.on_event @@ E_extension_event { tid; time_ns; ext }
+end
+
+let subscriber (consumer : event_consumer) : Sub.t =
+  Sub.Subscriber.Sub { st = consumer; callbacks = (module Callbacks) }