This paper presents findings from the first national randomized study of the impacts of charter schools on student achievement, which included 36 charter middle schools across 15 states. The paper compares students who applied and were admitted to these schools through randomized admissions lotteries with students who applied and were not admitted. It finds that, on average, charter middle schools in the study were neither more nor less successful than traditional public schools in improving student achievement. However, impacts varied significantly across schools and students, with positive impacts for more disadvantaged schools and students and negative impacts for the more advantaged. Appended are: (1) Baseline Characteristics of Treatment and Control Group Students in Full Sample; (2) Baseline Characteristics of Treatment and Control Group Students Included in Analysis of Year 2 Test Score Data; (3) Covariates Included in Impact Analysis Models; and (4) Impacts on State Proficiency Levels. (Contains 13 tables, 1 figure and 28 footnotes.)