diff --git a/core/shared/src/main/scala/works/iterative/core/auth/CurrentUser.scala b/core/shared/src/main/scala/works/iterative/core/auth/CurrentUser.scala
new file mode 100644
index 0000000..8543d10
--- /dev/null
+++ b/core/shared/src/main/scala/works/iterative/core/auth/CurrentUser.scala
@@ -0,0 +1,12 @@
+package works.iterative.core.auth
+
+import zio.*
+
+/** A service that provides the current user, if any.
+  */
+trait CurrentUser:
+  def currentUser: UIO[Option[UserId]]
+
+object CurrentUser:
+  def currentUser: URIO[CurrentUser, Option[UserId]] =
+    ZIO.serviceWithZIO(_.currentUser)