Since 2002, New York City has closed more than 20 underperforming public high schools, opened more than 200 new secondary schools, and introduced a centralized high school admissions process in which approximately 80,000 students a year indicate their school preferences from a wide-ranging choice of programs. At the heart of these reforms lie 123 new "small schools of choice" (SSCs)--small, academically nonselective, four-year public high schools for students in grades 9 through 12. Open to students at all levels of academic achievement and located in historically disadvantaged communities, SSCs were intended to be viable alternatives to the neighborhood high schools that were closing. The first step in New York City's high school admissions process is to require eighth-graders to select in rank order of priority up to 12 high schools that they want to attend; when an SSC has more applicants than spaces, the district uses a lottery-like process to randomly assign students to the SSC or to another school in the district. These lotteries provide the basis for an unusually large and rigorous study, supported by the Bill & Melinda Gates Foundation, of the effects of SSCs on students' academic achievement. This report presents encouraging findings from that study, providing clear and reliable evidence that, in roughly six years, a large system of small public high schools can be created and can markedly improve graduation prospects for many disadvantaged students. Specifically: (1) By the end of their first year of high school, 58.5 percent of SSC enrollees are on track to graduate in four years compared with 48.5 percent of their non-SSC counterparts, for a difference of 10.0 percentage points. These positive effects are sustained over the next two years; (2) By the fourth year of high school, SSCs increase overall graduation rates by 6.8 percentage points, which is roughly "one-third the size of the gap in graduation rates" between white students and students of color in New York City; and (3) SSCs' positive effects are seen for a broad range of students, including male high school students of color, whose educational prospects have been historically difficult to improve. Appendices include: (1) More About the Study's Research Design and Analysis; (2) Effects of Winning an SSC Lottery; (3) Baseline Characteristics of SSC Lottery Participants with Follow-up Data; (4) Imputing Outcomes for Students Who Are Lost Through Attrition; (5) Sources and Description of Data Collected for the Study; (6) Variation in the Fourth-Year Effects; (7) Small Schools of Choice in This Study; and (8) New York City Department of Education's New Secondary School Application, 2008. A bibliography is included. (Contains 30 tables, 5 figures, 5 boxes, and 110 footnotes.) [This paper was written with Corinne Herlihy and Collin F. Payne.