diff --git a/aio/content/guide/testing.md b/aio/content/guide/testing.md
index 2a0b292aa5..854b0afd95 100644
--- a/aio/content/guide/testing.md
+++ b/aio/content/guide/testing.md
@@ -618,6 +618,16 @@ It also generates an initial test file for the component, `banner-external.compo
   region="v1"
   header="app/banner/banner-external.component.spec.ts (initial)"></code-example>
 
+<div class="alert is-helpful">
+
+Because `compileComponents` is asynchronous, it uses 
+the [`async`](api/core/testing/async) utility 
+function imported from `@angular/core/testing`.
+
+Please refer to the [async](#async) section for more details.
+
+</div>
+
 #### Reduce the setup
 
 Only the last three lines of this file actually test the component