This paper describes source-code independent reverse engineering of dynamic web sites. The tool Revangie constructs a form-oriented analysis model solely from the usage of a web application. The recovered models can be, for example, exploited for the purpose of requirements engineering and load test development. Revangie can explore a given web application fully automatically or can passively record its usages. The collected data, i.e., data about screens, server-side programs, and system responsiveness, are analyzed in order to build a user interface model. The paper presents several useful classification techniques that can be utilized to reconstruct adequate models.