This paper presents extensions to Spec Explorer to automate the testing of software applications through their GUIs based on a formal specification in Spec . Spec Explorer, a tool developed at Microsoft Research, already supports automatic generation and execution of test cases for API testing, but requires that the actions described in the model are bound to methods in a .Net assembly. The tool described in this paper extends Spec Explorer to automate GUI testing: it adds the capability to gather information about the physical GUI objects that are the target of the user actions described in the model; and it automatically generates a .Net assembly with methods that simulate those actions upon the GUI application under test. The GUI modelling and the overall test process supported by these tools are described. The approach is illustrated with the Notepad application.