The school principal has long been recognized as a critical driver for school improvement and student success, but new research supported by The Wallace Foundation has found that principals have an even greater impact than previously stated. A synthesis of two decades of studies builds on previous research about how principals impact student outcomes and what characteristics and conditions lead to the greatest impact. The report, "How Principals Affect Students and Schools: A Systematic Synthesis of Two Decades of Research" (ED611009), updates the evidence base about school leadership, responds to changing policy contexts, and leverages recent improvements in research methodology. The report is a major contribution to the field as well as a critical resource for the revision of Learning Forward's Standards for Professional Learning. This research review describes the report methodology, and highlights findings and implications.