Adoption of statewide accountability systems for schools has been one of the most striking reforms in American education policy in the past 25 years. The change in focus away from inputs and processes and toward outcomes marks a dramatic shift in orientation. And yet we know little so far about how well these systems work. The lack of evidence on accountability is due in part to the way states have put these programs in place. States always have been the primary locus of education policy in the United States, adapting their programs to local circumstance and yielding a diverse array of programs across and within states. But, since each accountability program has invariably been implemented statewide at inception, it is impossible to ascertain the impact of any single state system. The variation in timing of implementation across states does nonetheless provide a way of estimating the impact of school accountability.