Adding to the growing debate and evidence base on the effects of charter schools, this evaluation was conducted in 36 charter middle schools in 15 states. It compares the outcomes of 2,330 students who applied to these schools and were randomly assigned by lotteries to be admitted (lottery winners) or not admitted (lottery losers) to the schools. Both sets of students were tracked over two years and data on student achievement, academic progress, behavior, and attitudes were collected. The study is the first large-scale randomized trial of the effectiveness of charter schools in varied types of communities and states. Among the key findings were that, on average, charter middle schools that held lotteries were neither more nor less successful than traditional public schools in improving math or reading test scores, attendance, grade promotion, or student conduct within or outside of school. Being admitted to a study charter school did significantly improve both students' and parents' satisfaction with school. Charter middle schools' impact on student achievement varied significantly across schools. Charter middle schools in urban areas--as well as those serving higher proportions of low-income and low achieving students--were more effective (relative to their nearby traditional public schools) than were other charter schools in improving math test scores. Some operational features of charter middle schools were associated with less negative impacts on achievement. These features include smaller enrollments and the use of ability grouping in math or English classes. There was no significant relationship between achievement impacts and the charter schools' policy environment. Because the study could only include charter middle schools that held lotteries, the results do not necessarily apply to the full set of charter middle schools in the U.S. Appended are: (1) Selecting the Charter School and Student Samples; (2) Calculation of Sample Weights; (3) Outcome Measures for the Impact Analysis; (4) Analytic Methods; (5) Supplemental Materials for Chapter III; (6) Supplemental Tables for Chapter IV; and (7) Supplemental Tables for Chapter V. (Contains 78 tables, 29 figures, and 164 footnotes.) [For the study snapshot of this full report, see ED510574.]