The success of the test-based accountability policy, No Child Left Behind (NCLB), designed to raise student achievement, will be strongly influenced by educators’ use of test-score data to guide school policy and classroom practice. This paper summarizes the data, providing insights for improving both the utility of test-score information at the classroom level, and the validity of large-scale testing systems. Because of the No Child Left Behind Act (NCLB), all states have implemented statewide testing programs in reading and mathematics for students in grade 3 to 8 and for one high school grade. However, most states have not examined the effects of testing on educational practice-an important omission, since a central assumption of NCLB is that the provision of high-quality information will promote improved educational decisionmaking. This study explores how teachers and principals use test score data, what features of its use are relevant to them, and what assistance is available to help them use the data for instructional decisionmaking. Using 2005 data, the study focuses on mathematics, using stratified random sampling to select 25 districts, and 100 schools per state (across three states) from within these districts. Within a single school, teachers have different impressions of the usefulness of the data. Over one-half of teachers surveyed reported that they used test results to identify topics for review and students who need attention. Most said that the test data were clear and easy to understand, and that they had received a variety of support for using the data. Most teachers relied on progress tests in instructional planning; they said that annual state test results were more useful for curriculum planning than for improving specific instructional practice. Periodic standardized progress testing or formative classroom assessments may be more useful than annual state tests. The variation observed in responses from teachers in different states suggests that state policy matters.