Teachers have the greatest school-based effect on the achievement of any child in their classrooms, but highly effective principals can positively affect the achievement of every student in their schools. The difference between a highly effective principal and an average one is equal to two-to-seven months of extra learning per year for each child in the school. The effect is greatest in high-poverty schools. This confirms the conclusions drawn from case studies of schools where student achievement "beats the odds": no low-achieving school has been turned around without a dynamic and effective principal. For better or for worse, school leaders shape the learning environment for teachers and students. Effective school leaders inspire their staff to improve and create conditions that enable high student achievement. Reforms to teacher preparation, licensure, evaluation, and compensation will be less successful if attention is not paid to building and improving school leadership. The role of principals has evolved, and policy needs to ensure they are prepared and supported to meet the challenges schools face today. This issue of re:VISION, part 5 of a special series on teacher effectiveness, examines the qualities of successful principals and offers considerations for policymakers who are working to improve their preparation and effectiveness. [For Part 1 of this series, see ED559386; for Part 2, see ED559387; for Part 3, see ED559381; and for Part 4, see ED559388.]