Graphical user interfaces (GUIs) are important parts of today's software and their correct execution is required to ensure the correctness of the overall software. A popular technique to detect defects in GUIs is to test them by executing test cases and checking the execution results. Test cases may either be created manually or generated automatically from a model of the GUI. While manual testing is unacceptably slow for many applications, our experience with GUI testing has shown that creating a model that can be used for automated test case generation is difficult.We describe a new approach to reverse engineer a model represented as structures called a GUI forest, event-flow graphs and an integration tree directly from the executable GUI. We describe "GUI Ripping", a dynamic process in which the software's GUI is automatically "traversed" by opening all its windows and extracting all their widgets (GUI objects), properties, and values. The extracted information is then verified by the test designer and used to automatically generate test cases. We present algorithms for the ripping process and describe their implementation in a tool suite that operates on Java and Microsoft Windows' GUIs.We present results of case studies which show that our approach requires very little human intervention and is especially useful for regression testing of software that is modified frequently. We have successfully used the "GUI Ripper" in several large experiments and have made it available as a downloadable tool.