Recent reforms in education emphasize the use of charter schools as a viable strategy to improve student achievement. It is, therefore, important to understand which types of charter schools are effective. I study this question utilizing longitudinal data covering all public school students in the large urban school district of Milwaukee which has a long history of charter schools. Using student fixed effects to deal with self-selection, I find that charter schools, on average, have no significant effect on student achievement. However, I show that this average effect masks important heterogeneity in the effectiveness of charter schools across types of charter schools. Charter schools with higher autonomy from the district in terms of financial budget, academic program, and hiring decisions, are effective. I show that students in these charter schools would read at a grade