School principals are critical to students' academic achievement, second only to teachers. They are the primary agents of change in their schools, fostering supportive learning environments that enhance student learning and teacher effectiveness. Moreover, they often design and oversee the initiatives and hiring to sustain the positive changes that they help bring about. There is a growing body of knowledge focused on the professional development of principals to help them be more effective in their key functions, such as coaching teachers and developing teachers' leadership skills. But there are few proven models for effective professional development of principals that improves student achievement. Because of this, developers, providers, and funders of such programs are short on evidence for how to align principals' professional learning and support with practices proven to help students achieve. To help address this gap, the United States Department of Education awarded two grants -- an Investing in Innovation (i3) grant awarded in 2014 and a Supporting Effective Educator Development (SEED) grant awarded in 2015 -- for the National Institute for School Leadership (NISL) to deliver its Executive Development Program (EDP) to approximately 900 school principals across five U.S. states. In addition to providing the EDP, NISL provided one-on-one in-person coaching. The RAND Corporation was the independent evaluator for both grants. RAND's two evaluations, the largest of their kind to date, resulted in several findings that can inform principal training and coaching programs going forward.