HttpModules sit in the ASP.NET processing pipeline and can listen for events during the processing lifecycle. Modules are good solutions when the behavior you want to achieve is orthogonal to the page processing. For instance, authentication, authorization, session state, and profiles are all implemented as HttpModules by the ASP.NET runtime. You can plug-in and remove these modules to add or discard their functionality. Here is a module to set the MasterPageFile property on every Page object.

public class MasterPageModule : IHttpModule