This Learning Policy Institute report describes key features of effective school leader preparation and professional development programs that enable principals to improve student and school outcomes. When investing in principal development, policymakers should ensure that programs reflect the evidence base for effective principal preparation by including strong partnerships; purposeful and targeted recruitment; cohort and network learning; problem-based learning methods; internships and on-the-job coaching by an expert; and curriculum and learning focused on improving instruction, creating collegial organizations, and using data to improve student achievement. The report underscores that strong principal leadership is necessary to implement new education initiatives related to curriculum, accountability, and social and emotional learning. Furthermore, it highlights that principals play a critical role in addressing teacher shortages by improving hiring practices, creating positive working conditions, and fostering collegial environments that can help improve teacher recruitment and retention.