The No Child Left Behind (NCLB) Act compelled states to design school accountability systems based on annual student assessments. The effect of this federal legislation on the distribution of student achievement is a highly controversial but centrally important question. This study presents evidence on whether NCLB has influenced student achievement based on an analysis of state-level panel data on student test scores from the National Assessment of Educational Progress (NAEP). The impact of NCLB is identified using a comparative interrupted time series analysis that relies on comparisons of the test-score changes across states that already had school accountability policies in place prior to NCLB and those that did not. Our results indicate that NCLB generated statistically significant increases in the average math performance of fourth graders (effect size = 0.23 by 2007) as well as improvements at the lower and top percentiles. There is also evidence of improvements in eighth-grade math achievement, particularly among traditionally low-achieving groups and at the lower percentiles. However, we find no evidence that NCLB increased fourth-grade reading achievement. (Contains 14 tables, 9 figures and 22 footnotes.)