The report is an update to the 2004 literature review, "How Leadership Influences Student Learning," which helped establish the importance of principals after concluding that school leadership was second only to classroom instruction in school-related impacts on student learning. The updated synthesis draws on 219 research studies about school leadership published in the 20 years since 2000, the latter end of the period covered by the earlier review. It was through their analysis of these studies that the authors reached their conclusions about principal effects on student achievement. The synthesis explores not only what the recent body of evidence says about principal impact, but also the demographic characteristics of principals in the second decade of the 21st century, the practices that define effective school leadership, and how principals can carry out those practices to promote equitable education in their schools. They also emphasize the necessity of principals embedding equity into these practices, as public schools serve growing numbers of students of color, students from low-income households, English learners, and students with disabilities. A related matter is the composition of the principal workforce, which, the authors find, has failed to keep pace with rapid changes in student demographics. The result is a significant "representation gap" between principals and students, a cause for concern in part because of research linking principal demographic diversity to better outcomes for students of color. [For "How Leadership Influences Student Learning," see ED485932.]