Web applications are increasingly prominent in society, serving a wide variety of user needs. Engineers seeking to enhance, test, and maintain these applications must be able to understand and characterize their interfaces. Third-party programmers (professional or end user) wishing to incorporate the data provided by such services into their own applications would also benefit from such characterization when the target site does not provide adequate programmatic interfaces. In this paper, therefore, we present methodologies for characterizing the interfaces to web applications through a form of dynamic analysis, in which directed requests are sent to the application, and responses are analyzed to draw inferences about its interface. We also provide mechanisms to increase the scalability of the approach. Finally, we evaluate the approach's performance on three well-known, non-trivial web applications.