Improving the nation's high schools—particularly those that are low-per-forming—involves challenges that are far easier to catalog than to surmount. In this article, the authors identify a handful of promising approaches that can help to achieve the goal that all students will graduate from high school well-prepared for further learning, successful careers, and engaged citizenship. The authors explain the theories that drive these high school improvement models, review evidence of their effectiveness to date, and suggest what it will take to make them work well. The authors stress that no single school improvement model or approach, no matter how powerful, can ensure the success of all students or schools. The reasons for poor performance are complex and determined by multiple intersecting personal, community, and organizational factors. These inequities have very real consequences for schooling and makes the job of improvement that much more challenging. The article concludes with a set of recommendations for policymakers, researchers, and sponsors of research to enhance the evidence base and increase our knowledge of how high schools and high school success outcomes can improve over time.