This report from the Learning Policy Institute and the National Association of Secondary School Principals (NASSP) explains the importance of school principals, reasons they leave, and policy implications for principal retention and effectiveness. While teacher shortages are an increasingly critical issue in the United States, a lesser known but equally important shortage is also hampering the country’s efforts to provide quality educational opportunities for students—principal shortages. As disruption from the COVID-19 pandemic continues, school leaders who are skilled at supporting student learning and well-being are even more critical, and yet nearly one in five principals leave their schools each year, and the average tenure of a principal is only about 4 years. These numbers are higher in the under-resourced schools that tend to serve the highest populations of students of color and students from low-income families who will be hit hardest by the pandemic. This report is the fourth in a series focusing on principals.