This article presents findings from a lottery-based study of the impacts of a broad set of 33 charter middle schools across 13 states on student achievement. To estimate charter school impacts, we compare test score outcomes of students admitted to these schools through the randomized admissions lotteries with outcomes of applicants who were not admitted. We find that impacts varied considerably across schools and students, with more positive impacts for more disadvantaged schools and students and more negative impacts for the more advantaged. On average across the schools in the study, the impacts of charter middle schools on student achievement were negative but not statistically significant, regardless of whether we examined the impact of the offer of admission or actual attendance at these schools.