Methods, systems, and apparatus, including an apparatus for analyzing content that is displayed within an application (e.g., a native application) on a client device and making decisions regarding the presentation of content within the application. In one aspect, a process includes identifying an event occurring within the application. The event can be indicative of a display of content by the application. The content to be displayed is received from the application. Information representative of the content is sent to a server. A response is received from the server. The response is indicative of the suitability of the content for display within the application.