Test-based accountability systems are now a central feature of U.S. education policy. Accountability systems are implemented as a way to improve 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 states have developed offer very different incentives. State accountability systems differ in a variety of important ways. We concentrate on the school activities that are the focus of attention (outcomes or a mixture of inputs and process measures), the scope of the measures included, and the design of reporting and incentives. We then consider the available evidence on impacts of different systems. 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. We conclude with new evidence about the impacts of accountability systems on achievement and on special education placement rates.