School accountability—the process of evaluating school performance on the basis of student performance measures—is increasingly prevalent around the world. In the United States, accountability has become a centerpiece of both Democratic and Republican federal administrations' education policies. This chapter reviews the theory of school-based accountability, describes variations across programs, and identifies key features influencing the effectiveness and possible unintended consequences of accountability policies. The chapter then summarizes the research literature on the effects of test-based accountability on students and teachers, concluding that the preponderance of evidence suggests positive effects of the accountability movement in the United States during the 1990s and early 2000s on student achievement, especially in math. The effects on teachers and on students' long-run outcomes are more difficult to judge. It is also clear that school personnel respond to accountability in both positive and negative ways, and that accountability systems run the risk of being counter-productive if not carefully thought out and monitored.