States are engaged in serious efforts to raise student achievement, paying particular attention to narrowing the achievement gap between high- and low-performing students that has persisted in public schools for decades. The achievement gap is particularly troubling because, all too often, those at low achievement levels are minority, special education, or limited-English-proficient students. The federal "No Child Left Behind" (NCLB) act has given added impetus to efforts to close the achievement gap. States are now testing all students in specific grades in mathematics and reading, using standardized state assessments aligned with state academic standards. Efforts to close the achievement gap are as diverse as the 50 states, and many educators are struggling to implement school improvement strategies and find adequate and appropriate measurements to see if these strategies are working. "NCLB" offers a useful framework for thinking about the kinds of data needed to identify students having difficulty meeting standards and to document school progress in closing the achievement gap over time. Good data make it possible to arrive at fair, accurate judgments about student and school performance and to make better decisions about how to deploy resources to improve schools. This policy brief describes the elements of a comprehensive state student data system that can provide richer and more meaningful information of use to policymakers, educators, parents, and the public. (Contains 4 figures and 2 footnotes.) [This paper was written with Chrys Dougherty.]