In the early 1990s, states began establishing standards in academic subjects backed by test-based accountability systems to see that the standards were met. Incentives were implemented for schools and students based on pupil test scores. These early accountability systems paved the way for passage of landmark federal legislation, the No Child Left Behind Act, in 2002. The first section of this paper describes what is known so far about the effects of accountability systems on student achievement. After reviewing the evidence, I conclude that evaluations of accountability systems are generally quite positive. The potential for unintended consequences cannot be ruled out, but the harms documented to date appear temporary and malleable. The second section examines the political threats that arise when accountability systems are implemented. I argue that sanctions on low-performing students and schools, which are just now beginning to be implemented under No Child Left Behind, motivate much of the opposition to accountability, but the opposition is also sustained by how school systems are organized and governed. If future research reveals how struggling students and failing schools can improve, the findings will go a long way in dissipating the political opposition to accountability. [In: Ravitch, Diane, Ed., Brookings Papers on Education Policy, 2005. Washington, DC: Brookings Institution Press, 2005. p7-45. See ED488830.]