Principals are often seen as the primary agents of change to improve student achievement in their schools. Yet the role of the principal is complex, and there are many ways that principals might potentially influence classroom instruction and student learning. What matters most? Researchers used data from hundreds of schools to learn how principals were most effective at achieving higher learning gains on standardized tests. Then, they visited 12 schools, interviewing principals and teachers, to see firsthand what principals in schools with improving learning gains were doing that principals in schools without improving learning gains were not.