In the first decade of the 21st century, the New York City (NYC) Department of Education implemented a set of large-scale and much debated high school reforms, which included closing large, low-performing schools, opening new small schools, and extending high school choice to students throughout the district. The school closure process was the most controversial of these efforts. Yet, apart from the general sense that school closures are painful, there has never been a rigorous assessment of their impact in NYC. To begin to fill this gap, the Research Alliance undertook a study of the 29 low-performing high schools that were designated for closure in New York City between 2002 and 2008, looking particularly at the impact of these closures on students' academic performance, attendance, and mobility. This report presents the full results of the analyses. Key findings include: (1) The schools designated for closure were, in fact, among the lowest performing in the City, based on a composite of 10 performance indicators averaged over four years leading up to the closure decision; (2) Closures had little impact, positive or negative, on the academic outcomes of students who were enrolled during the phaseout process; (3) The phaseout process did increase student mobility, largely through transfers to other New York City high schools, rather than transfers to other districts or students dropping out; and (4) Closing high schools produced meaningful benefits for future students--i.e., middle schoolers who had to choose another high school because the school they likely would have attended was closing. The findings from this study, together with strong evidence on the positive effects of NYC's new small schools, provide support for the strategic use of school closures in the context of this kind of multi-dimensional high school reform effort. While this study shows clear gains for students in the wake of a closure, these students still did not fare well. Dramatic actions like school closures have helped set the system on a more positive trajectory. However, additional strategies are likely needed for the challenges of today and the future. [For the brief, see ED573428. For the technical appendices, see ED573427.]