There are almost 3,000 charter schools operating in the United States. In Arizona about 500 charters enroll about 74,000 students. The question of whether charter schools "work" is, therefore, of considerable interest to policymakers and the public nationally and in Arizona. To charter school proponents, the fact that charters exist is sometimes taken as evidence of their value because they believe charters create competition for students that push traditional public schools toward improvement. No wonder the release of a report by the American Federation of Teachers last month showing that National Assessment of Educational Progress data revealed charter school students were performing less well than their traditional public school counterparts caused an explosion of outrage among charter supporters. In this essay, the author discusses how several studies have failed to provide a complete and clear overall picture of the effectiveness of charter schools. What is needed are sound, comprehensive, longitudinal data that allow individuals to track the performance of charter schools over time.