In some implementations, a computing device can use notification extensions for applications to preprocess a notification received by the computing device and present content associated with a notification. For example, applications installed on the computing device can provide a service extension for preprocessing the notification. When a notification associated with the application is received, the notification system on the computing device can invoke the service extension for the application to preprocess the notification (e.g., to download additional content, encrypt/decrypt notification data, etc.). Applications can provide content extensions to present notification content. For example, a content extension associated with the application can be invoked by the notification system to present media content (or other content) associated with a received notification in an application specific manner.