Test based accountability systems are now a central feature of U.S. education policy. Accountability systems are implemented as a way of improving student outcomes through new, highly visible incentives. In analyzing the effectiveness of such state systems, the correct comparison is not accountability versus no accountability but the differential effects related to the type of system that is employed. The alternative systems that have developed have very different incentives. While research on the outcomes of accountability systems is growing rapidly, it still represents a young and highly selective body of work. The existing research suggests that schools definitely respond to the incentives of accountability systems, but the form and strength of such responses is highly variable. This paper characterizes the incentives of different systems and reviews the existing evidence about outcomes.