No Child Left Behind (NCLB) is a new federal law in the U.S. that requires states (if they wish to receive federal education funds) to set standards for student learning, design assessments to measure progress towards standards, and to hold districts and schools accountable for results. Accountability through high-stakes standardized testing is the latest development in a long move towards standards-based reform in U.S. education, a reform process that has enjoyed bipartisan support. It occurs in the context of what has been a “loosely coupled” system with little formal supervision and no national testing. NCLB confronts four dilemmas that any system of testing and accountability must address: It sets high standards; uses absolute targets to assess progress; accepts the narrowness of standardized tests in only two subjects as the price to be paid for clear and specific standards; and attempts (perhaps with limited success) to identify mechanisms to ensure equal opportunity to meet the standards. As a result of this approach, NCLB has galvanized the attention of educators and motivated teachers and students to improve; but it also leads to disproportionate sanctions for schools serving disadvantaged populations, mixes up effective and ineffective schools, and poses an unrealistic time frame for success.