Recent educational reforms have promoted accountability systems which attempt to identify teacher effects on student outcomes and hold teachers accountable for producing learning gains. But in the complex world of classrooms, it may be difficult to attribute 'success' or 'failure' to teachers. In this timely collection, leading education scholars challenge market-based models of school improvement and argue that merely holding teachers accountable for scores on end-of-the-year exams will not lead to educational improvement. The authors show why, in addition to test performance, a close examination of instructional processes and school context are needed in order to truly understand teacher effects and improve learning in our nation's classrooms.