Abstract: An emulating system is provided for testing a computer application for use with a target computing system. The emulating system emulates the input/output behavior of the target system. The emulating system has access to a data source, which contains a plurality of requests acceptable to the target system and a plurality of responses. Each acceptable request is associated with a response that describes the expected behavior of the target system in response to the acceptable request. After communication between the emulating system and the application is established, the emulating system responds to a request from the application by obtaining a response associated with the request from the data source and responding as described by the response associated with the request.