diff --git a/dune-project b/dune-project
index 478434af..63684022 100644
--- a/dune-project
+++ b/dune-project
@@ -27,6 +27,7 @@
   (ocaml
    (>= "4.08"))
   ptime
+  hmap
   ambient-context
   (odoc :with-doc)
   (alcotest :with-test)
diff --git a/opentelemetry.opam b/opentelemetry.opam
index cae04f6a..756e1860 100644
--- a/opentelemetry.opam
+++ b/opentelemetry.opam
@@ -16,6 +16,7 @@ depends: [
   "dune" {>= "2.9"}
   "ocaml" {>= "4.08"}
   "ptime"
+  "hmap"
   "ambient-context"
   "odoc" {with-doc}
   "alcotest" {with-test}
diff --git a/src/core/dune b/src/core/dune
index f3671f64..f442b5ac 100644
--- a/src/core/dune
+++ b/src/core/dune
@@ -3,5 +3,5 @@
  (synopsis "API for opentelemetry instrumentation")
  (flags :standard -warn-error -a+8)
  (libraries opentelemetry.proto ambient-context ptime ptime.clock.os pbrt threads
-   opentelemetry.atomic)
+   opentelemetry.atomic hmap)
  (public_name opentelemetry))
diff --git a/src/core/opentelemetry.ml b/src/core/opentelemetry.ml
index 165e7e3a..e26e0391 100644
--- a/src/core/opentelemetry.ml
+++ b/src/core/opentelemetry.ml
@@ -326,6 +326,11 @@ end = struct
   let pp fmt t = Format.fprintf fmt "%s" (to_hex t)
 end
 
+(** Hmap key to carry around a {!Trace_id.t}, to remember what the current
+    trace is.
+    @since NEXT_RELEASE *)
+let k_trace_id : Trace_id.t Hmap.key = Hmap.Key.create ()
+
 (** Unique ID of a span. *)
 module Span_id : sig
   type t
@@ -479,6 +484,11 @@ end = struct
     | Error msg -> invalid_arg @@ spf "invalid w3c trace context: %s" msg
 end
 
+(** Hmap key to carry around a {!Span_ctx.t}, e.g. to remember what the current
+    parent span is.
+    @since NEXT_RELEASE *)
+let k_span_ctx : Span_ctx.t Hmap.key = Hmap.Key.create ()
+
 (** {2 Attributes and conventions} *)
 
 module Conventions = struct