diff --git a/docs/reference/cainjector.rst b/docs/reference/cainjector.rst
index e43301368..1a7626e4d 100644
--- a/docs/reference/cainjector.rst
+++ b/docs/reference/cainjector.rst
@@ -1,3 +1,12 @@
 =====================
 cainjector controller
 =====================
+
+The cainjector controller injects a Certificate into the ``caBundle`` field
+of ValidatingWebhookConfiguration, MutatingWebhookConfiguration or
+APIService resources annotated with:
+
+* ``cert-manager.io/inject-apiserver-ca: "true"``  
+  Injects the cluster CA.
+* ``cert-manager.io/inject-ca-from: <NAMESPACE>/<CERTIFICATE>``  
+  Injects the CA from the specified :doc:`certificate </reference/certificates>`.