For decades, New York City's high school graduation rates hovered at or below 50 percent. In attempt to turn around these disappointing results, the NYC Department of Education enacted a series of large-scale reforms, including opening hundreds of new "small schools of choice" (SSCs). Recent research by MDRC has shown that these schools have had large and sustained positive effects on students' graduation rates and other outcomes. How have they done it? What decisions--made by the educators who created, supported, and operated these schools--have been critical to their success? What challenges do these schools face as they try to maintain that success over time? The Research Alliance set out to answer these questions, conducting in-depth interviews with teachers and principals in 25 of the most highly effective SSCs. Educators reported three features as essential to their success: (1) Personalization, which was widely seen as the most important success factor. This includes structures that foster strong relationships with students and their families, systems for monitoring student progress--beyond just grades and test scores, and working to address students' social and emotional needs, as well as academic ones. (2) High expectations--for students and for educators--and instructional programs that are aligned with these ambitious goals. (3) Dedicated and flexible teachers, who were willing to take on multiple roles, sometimes outside their areas of expertise. The findings, presented in this report, paint a picture of how these features were developed in practice. The report also describes challenges these schools face and outlines lessons for other schools and districts that can be drawn from the SSCs' experience. These include the need to avoid teacher burnout, improving the fit between schools and external partners, and expanding current notions of accountability. [For the executive summary of this report, see ED556054; for the technical appendices, see ED556056.]