In low-performing public high schools in U.S. cities, high proportions of students drop out, students who stay in school typically do not succeed academically, and efforts to make substantial reforms often meet with little success. The Talent Development High School model is a comprehensive school reform initiative that has been developed to address these challenges. Targeting some of the most troubled schools in the country, the model seeks to raise the expectations of teachers and students and to prepare all students for postsecondary education and employment. MDRC, a nonpartisan, nonprofit education and social policy research organization, conducted an independent, third-party evaluation of Talent Development. This rigorous evaluation focuses on the first five high schools to begin using the model in the School District of Philadelphia. The evaluation follows 20 cohorts of ninth-grade students for up to four years of high school using a comparative interrupted time series research design. Appended are: (1) Tables for First-Time Ninth-Grade Students; and (2) Tables for Repeating Ninth-Grade Students. (Contains 16 tables, 9 figures, and 3 boxes.)[Dissemination of MDRC publications is also supported by Starr Foundation.]