In this paper, we present an overview of HUI Analyzer, a tool intended for automating usability testing. The tool allows a user interface’s expected and actual use to be captured unobtrusively, with any mismatches indicating potential usability problems being highlighted. HUI Analyzer also supports specification and checking of assertions governing a user interface’s layout and actual user interaction. Assertions offer a low cost means of detecting usability defects and are intended to be checked iteratively during a user interface’s development. Hotspot analysis is a feature that highlights the relative use of GUI components in a form. This is useful in informing form layout, for example to collocate heavily used components thereby reducing unnecessary scrolling or movement. Based on evaluation, we have found HUI Analyzer’s performance in detecting usability defects to be comparable to conventional formal user testing. However the time taken by HUI Analyzer to automatically process and analyze user interactions is much less than that for formal user testing.