The Custom Activity Framework is a set of classes that simplify the development of custom activities. In this first version the base classes are designed for building activities that need to listen for external events. Examples of external events might be a file being created on the local file system, data being received in a queue, or the asynchronous completion of a web service call. To demonstrate how the base classes can be used, this package includes the FileSystemEvent activity that derives from the InputActivity base class in the Custom Activity Framework.