Charter schools have moved from being widely viewed as a marginal force in public education reform to taking on a central role in our national, state, and local debates around improving education. And a growing number of policymakers— including superintendents, mayors, governors, and even U.S. Secretary of Education Arne Duncan—are using chartering as a critical tool to drive school improvement and meet the needs of underserved children. These developments create new demands on authorizers to strategically manage their portfolios by effectively monitoring the quality of the schools they have chartered, closing low-performing schools, and fostering the replication and growth to scale of high performers.