The No Child Left Behind (NCLB) Act has 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). This study identifies the impact of NCLB by relying 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. Results indicate that NCLB generated statistically significant increases in the average math performance of 4th graders (effect size = 0.22 by 2007) as well as improvements at the lower and top percentiles. However, the authors do not find consistent evidence that NCLB generated similarly broad improvements in reading achievement or achievement among 8th graders. (Contains 11 tables and 6 footnotes.) [This paper was presented at the "NCLB: Emerging Findings Research Conference" at the Urban Institute, Washington, D.C. on August 12, 2009.]