Context: GUI testing is system testing of a software that has a graphical-user interface (GUI) front-end. Because system testing entails that the entire software system, including the user interface, be tested as a whole, during GUI testing, test cases-modeled as sequences of user input events-are developed and executed on the software by exercising the GUI's widgets (e.g., text boxes and clickable buttons). More than 230 articles have appeared in the area of GUI testing since 1991. Objective: In this paper, we study this existing body of knowledge using a systematic mapping (SM). Method: The SM is conducted using the guidelines proposed by Petersen et al. We pose three sets of research questions. We define selection and exclusion criteria. From the initial pool of 230 articles, published in years 1991-2011, our final pool consisted of 136 articles. We systematically develop a classification scheme and map the selected articles to this scheme. Results: We present two types of results. First, we report the demographics and bibliometrics trends in this domain, including: top-cited articles, active researchers, top venues, and active countries in this research area. Moreover, we derive the trends, for instance, in terms of types of articles, sources of information to derive test cases, types of evaluations used in articles, etc. Our second major result is a publicly-accessible repository that contains all our mapping data. We plan to update this repository on a regular basis, making it a ''live'' resource for all researchers. Conclusion: Our SM provides an overview of existing GUI testing approaches and helps spot areas in the field that require more attention from the research community. For example, much work is needed to connect academic model-based techniques with commercially available tools. To this end, studies are needed to compare the state-of-the-art in GUI testing in academic techniques and industrial tools.