As the nation has embraced the need to graduate every student ready for college and careers, high school reform has emerged at the top of the education agenda. Many local and state leaders are implementing strategies to address low performance and close achievement gaps. As federal policymakers look ahead to opportunities to support this work--including through the upcoming reauthorization of the Elementary and Secondary Education Act (ESEA)--they are eagerly looking to districts that have been engaged in major reform to understand the implications for supporting and encouraging these reforms at scale. One extremely relevant case study is New York City (NYC)--the nation's largest and most diverse school district--where Mayor Michael Bloomberg and Schools Chancellor Joel Klein have prioritized redesigning high schools and improving outcomes as part of a districtwide reform effort. Children First, as the reforms are collectively known, has caught the attention of advocates, policymakers, and educators across the country for the breadth of the changes implemented as well as for its preliminary indications of success in improving student outcomes and closing achievement gaps. Most promising is the steady increase of four-year graduation rates by as much as 15 points since 2002, after a decade of stagnation. Given its size, demographics, and history, NYC faces some of the greatest challenges that federal policy seeks to address, including high-risk and diverse student populations, chronic low performance, stubborn inequities, a history of entrenched bureaucracy, and some community resistance to change. In addition, the city has implemented many of the reform practices that have dominated federal policy discussions, including turning around or replacing low-performing high schools, using data to inform instruction, and improving education options for at-risk students. This brief describes the theory of action underlying the efforts of the New York City Department of Education (NYCDOE) and some of the specific strategies it has employed to improve high schools. (Contains 5 footnotes and 44 endnotes.)