diff --git a/rmw/include/rmw/event_callback_type.h b/rmw/include/rmw/event_callback_type.h
new file mode 100644
index 00000000..8833b49f
--- /dev/null
+++ b/rmw/include/rmw/event_callback_type.h
@@ -0,0 +1,52 @@
+// Copyright 2021 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef RMW__EVENT_CALLBACK_TYPE_H_
+#define RMW__EVENT_CALLBACK_TYPE_H_
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+/// Common event callback type signature.
+/**
+ * Event callbacks of this type can be called in various scenarios, e.g.
+ * data becomes available on a subscription, a QoS event has occurred, or
+ * something similar.
+ *
+ * The user_data argument is given by the user when registering the callback,
+ * and is given back to the callback each time so it can have associated,
+ * user-defined state.
+ *
+ * The number_of_events argument indicates the number of events since the
+ * callback was called.
+ * This is most often 1, but can be > 1 when events occur before the callback
+ * is registered.
+ * It should never be 0.
+ *
+ * \sa rmw_subscription_set_on_new_message_callback()
+ * \sa rmw_service_set_on_new_request_callback()
+ * \sa rmw_client_set_on_new_response_callback()
+ * \sa rmw_event_set_callback()
+ */
+typedef void (* rmw_event_callback_t)(const void * user_data, size_t number_of_events);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RMW__EVENT_CALLBACK_TYPE_H_
diff --git a/rmw/include/rmw/rmw.h b/rmw/include/rmw/rmw.h
index bb224e2c..c98011e5 100644
--- a/rmw/include/rmw/rmw.h
+++ b/rmw/include/rmw/rmw.h
@@ -98,12 +98,14 @@ extern "C"
 #include "rosidl_runtime_c/service_type_support_struct.h"
 #include "rosidl_runtime_c/sequence_bound.h"
 
+#include "rmw/event.h"
 #include "rmw/init.h"
+#include "rmw/event_callback_type.h"
 #include "rmw/macros.h"
+#include "rmw/message_sequence.h"
 #include "rmw/publisher_options.h"
 #include "rmw/qos_profiles.h"
 #include "rmw/subscription_options.h"
-#include "rmw/message_sequence.h"
 #include "rmw/types.h"
 #include "rmw/visibility_control.h"
 
@@ -2995,6 +2997,146 @@ RMW_WARN_UNUSED
 rmw_ret_t
 rmw_set_log_severity(rmw_log_severity_t severity);
 
+/// Set the on new message callback function for the subscription.
+/**
+ * This API sets the callback function to be called whenever the
+ * subscription is notified about a new message.
+ *
+ * This callback is called for each new message received by the subscription.
+ * If messages arrive before the callback is registered, the number_of_events
+ * argument given to the callback may be > 1.
+ *
+ * The callback may be called from a thread that the rmw implementation
+ * created, rather than a thread owned by the user, i.e. some thread other
+ * than user owned threads calling rmw functions such as rmw_wait() or
+ * rmw_publish().
+ *
+ * This function is thread-safe.
+ * This is required of the rmw implementation because the callback may be called
+ * from any middleware thread, and this function could be called by the user
+ * at any time.
+ *
+ * \param[in] subscription The subscription on which to set the callback
+ * \param[in] callback The callback to be called when new messages arrive
+ * \param[in] user_data Given to the callback when called later, may be NULL
+ * \return `RMW_RET_OK` if successful, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if `subscription` or `callback` is NULL, or
+ * \return `RMW_RET_UNSUPPORTED` if the API is not implemented in the dds implementation
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_subscription_set_on_new_message_callback(
+  rmw_subscription_t * subscription,
+  rmw_event_callback_t callback,
+  const void * user_data);
+
+/// Set the on new request callback function for the service.
+/**
+ * This API sets the callback function to be called whenever the
+ * service is notified about a new request.
+ *
+ * This callback is called for each new request received by the service.
+ * If requests arrive before the callback is registered, the number_of_events
+ * argument given to the callback may be > 1.
+ *
+ * The callback may be called from a thread that the rmw implementation
+ * created, rather than a thread owned by the user, i.e. some thread other
+ * than user owned threads calling rmw functions such as rmw_wait() or
+ * rmw_send_request().
+ *
+ * This function is thread-safe.
+ * This is required of the rmw implementation because the callback may be called
+ * from any middleware thread, and this function could be called by the user
+ * at any time.
+ *
+ * \param[in] service The service on which to set the callback
+ * \param[in] callback The callback to be called when new requests arrive
+ * \param[in] user_data Given to the callback when called later, may be NULL
+ * \return `RMW_RET_OK` if callback was set to the listener, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if `service` or `callback` is NULL, or
+ * \return `RMW_RET_UNSUPPORTED` if the API is not implemented in the dds implementation
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_service_set_on_new_request_callback(
+  rmw_service_t * service,
+  rmw_event_callback_t callback,
+  const void * user_data);
+
+/// Set the on new response callback function for the client.
+/**
+ * This API sets the callback function to be called whenever the
+ * client is notified about a new response.
+ *
+ * This callback is called for each new response received by the client.
+ * If responses arrive before the callback is registered, the number_of_events
+ * argument given to the callback may be > 1.
+ *
+ * The callback may be called from a thread that the rmw implementation
+ * created, rather than a thread owned by the user, i.e. some thread other
+ * than user owned threads calling rmw functions such as rmw_wait() or
+ * rmw_take_response().
+ *
+ * This function is thread-safe.
+ * This is required of the rmw implementation because the callback may be called
+ * from any middleware thread, and this function could be called by the user
+ * at any time.
+ *
+ * \param[in] client The client on which to set the callback
+ * \param[in] callback The callback to be called when new responses arrive
+ * \param[in] user_data Given to the callback when called later, may be NULL
+ * \return `RMW_RET_OK` if callback was set to the listener, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if `client` or `callback` is NULL, or
+ * \return `RMW_RET_UNSUPPORTED` if the API is not implemented in the dds implementation
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_client_set_on_new_response_callback(
+  rmw_client_t * client,
+  rmw_event_callback_t callback,
+  const void * user_data);
+
+/// Set the callback function for the event.
+/**
+ * This API sets the callback function to be called whenever the
+ * event is notified about a new instance of the event.
+ *
+ * For example, this could be called when incompatible QoS is detected, or
+ * a deadline is missed, or any other QoS event.
+ *
+ * This callback is called for each new event that occurs for this rmw_event_t
+ * instance.
+ * If events occur before the callback is registered, the number_of_events
+ * argument given to the callback may be > 1.
+ *
+ * The callback may be called from a thread that the rmw implementation
+ * created, rather than a thread owned by the user, i.e. some thread other
+ * than user owned threads calling rmw functions such as rmw_wait() or
+ * rmw_publish().
+ *
+ * This function is thread-safe.
+ * This is required of the rmw implementation because the callback may be called
+ * from any middleware thread, and this function could be called by the user
+ * at any time.
+ *
+ * \param[in] event The event on which to set the callback
+ * \param[in] callback The callback to be called when new events occur
+ * \param[in] user_data Given to the callback when called later, may be NULL
+ * \return `RMW_RET_OK` if callback was set to the listener, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if `event` or `callback` is NULL, or
+ * \return `RMW_RET_UNSUPPORTED` if the API is not implemented in the dds implementation
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_event_set_callback(
+  rmw_event_t * event,
+  rmw_event_callback_t callback,
+  const void * user_data);
+
 #ifdef __cplusplus
 }
 #endif