This research report from the Wallace Foundation 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 high-quality research studies about school leadership published in the 20 years since 2000. Among them are six studies published since 2012 that examine principal impact by taking advantage of school sand principal longitudinal data unavailable 20 years ago. The authors note that studies using new data and methods show that the importance of principals may not have been stated strongly enough in earlier work, given the magnitude and scope of principals’ impacts on students and schools.