In this study, we analyze the achievement of students in the charter school system with the largest enrollment in the United States. The charter is held by the Chicago Charter Schools Foundation, which is a charitable organization, and the system is called the Chicago International Charter School ("CICS"). Currently, CICS includes seven schools with a total enrollment of 4,430 students in fall 2003. Five of the schools are primary schools (serving some or all of grades pre-kindergarten to eight), one of the schools serves all grades from kindergarten to twelve, and the final school serves grades nine through twelve. CICS has its own management organization run its high school and one of its primary schools. Four CICS primary schools are managed by a non-profit organization, American Quality Schools. The final (kindergarten through grade twelve) school is managed by Edison Schools, which is for-profit. Thus, CICS represents a good part of the spectrum of school models and management models among charter schools. In addition, CICS students are fairly typical of students who attend charter schools located in highly urban areas: its students are mainly black or Hispanic and most participate in the federal Free and Reduced Lunch Program, which means that they come from households with incomes below 185 percent of the federal poverty line. CICS schools currently have at least 1200 more applicants than they can admit, given their space. Each school uses randomized lotteries to admit applicants when oversubscribed. CICS provides us with an excellent environment for investigating charter schools' effects, not only because it is representative of a key part of the charter school sector in the United States, but also because access to its data and CPS data allow us to examine both lotteried-in and lotteried-out applicants. The data also let us address issues like non-compliance, attrition, and reapplication. Finally, we are able to examine whether the charter school treatment effect depends on a student's grade level, initial achievement, and the number of years the school has been operating.