Methods and apparatus, including computer program products, for implementing extensions for computer program applications. A program according to one aspect of the invention has instructions to receive an application call to an extension method defined in an extension object definition, the extension object definition having associated extension object implementations, the extension object implementations providing extension method implementations of the extension method; instructions operable to obtain results by calling multiple extension method implementations of the extension method; and instructions operable to respond to the application call using the results obtained from the extension method implementations. A method according to another aspect includes the steps of deploying multiple extension method implementations for an extension method; and executing the multiple extension method implementations to determine the results of calling the extension method.