This article describes the results of a three-year investigation into cheating by school personnel. The goals of this research were to measure the prevalence of cheating by teachers and administrators and to analyze the factors that predict cheating. Using data on test scores and student records from the Chicago Public Schools, the authors developed a statistical algorithm to identify classrooms where cheating was suspected. This method depends on two hallmarks of potential cheating: unexpected fluctuations in students' test scores and unusual patterns of answers for students within a classroom. The results of this study demonstrate the value of statistical analysis to school districts interested in catching cheaters or deterring future cheating. While evidence of cheating is sometimes used to impugn high-stakes testing programs, the results actually show that explicit cheating by school personnel is not likely to be a serious enough problem by itself to call into question high-stakes testing, both because the most egregious forms of cheating are relatively rare and, more important, because cheating could be virtually eliminated at a relatively low cost through the implementation of proper safeguards, such as those used by the Educational Testing Service on the SAT or GRE exams. (Contains 2 figures.)