Under the No Child Left Behind Act, states have been required to set minimum proficiency standards that virtually all students must meet by 2014. Sanctions of increasing severity are to be applied to schools that fail to meet interim targets, known as Adequate Yearly Progress (AYP). The authors examine the effect of this legislation using longitudinal, student-level test score data from seven states (N greater than 2,000,000) between 2002-03 and 2005-06 school years. This paper addresses the following research questions: (1) Has NCLB increased achievement among lower-performing students?; (2) Have these gains come at the expense of students that are already proficient or that are far below the proficiency target? Identification is achieved by exploiting the fact that in the early years of NCLB, not all grades counted for purposes of determining AYP. The estimate of the NCLB effect is therefore based on a comparison of outcomes in high-stakes vs. low-stakes years. The authors find consistent evidence of an achievement trade-off in the hypothesized direction, though the effects on any given student are not large. Unlike some other researchers, they find mixed evidence at best that students far below the proficient level have been harmed by NCLB; indeed, at higher grade levels they appear to have benefited. Effects of NCLB on efficiency, while positive, appear to be modest. NWEA Score Alignment Studies is appended. (Contains 12 figures, 7 tables and 16 footnotes.) [This paper was presented at the "NCLB: Emerging Findings Research Conference" at the Urban Institute, Washington, D.C. on August 12, 2009.]