In this article, the authors tackle the question of whether the No Child Left Behind (NCLB) Act has affected student achievement since its enactment in 2002. They present new research that takes on the challenge of distinguishing the law's effects from the myriad other factors at work over the past eight years. The results suggest that its consequences have been mixed. The authors find that the accountability provisions of NCLB generated large and statistically significant increases in the math achievement of 4th graders and that these gains were concentrated among African American and Hispanic students and among students who were eligible for subsidized lunch. They find smaller positive effects on 8th-grade math achievement. These effects are concentrated at lower achievement levels and among students who were eligible for subsidized lunch. They do not, however, find evidence that NCLB accountability had any impact on reading achievement among either 4th or 8th graders. The mixed results presented in this article pose difficult but important questions for policymakers considering whether to "end" or "mend" NCLB. The evidence of substantial and almost universal gains in math is undoubtedly good news for advocates of NCLB. But the lack of any effect in reading, and the fact that the policy appears to have generated only modestly larger impacts among disadvantaged subgroups in math (and thus made only minimal headway in closing achievement gaps), suggests that the impact of NCLB has fallen short of its extraordinarily ambitious goals. (Contains 3 figures.)