The Every Student Succeeds Act (ESSA) presents a renewed focus on school leadership and acknowledges the importance of school principals to school improvement and effective instruction (Public Law No. 114-95, 2015). The act allows states and districts to use federal funds for activities targeting the quality of school principals and other school leaders. ESSA repeatedly calls for the use of "evidence-based" activities, strategies, and interventions (Public Law No. 114-95, 2015). The RAND Corporation conducted a synthesis of the evidence base on school leadership efforts to inform the use of school leadership activities and interventions under ESSA. This report is intended to help federal, state, and district education policymakers understand and implement efforts to improve school leadership that are consistent with ESSA. Herein, the authors first offer an overview of the ways in which school leadership may affect outcomes of interest and then describe how school leadership is addressed by ESSA funding streams and statutory provisions. The key questions for this topic are: (1) What is the evidence that school leadership matters for school improvement? and (2) What school leadership--improvement activities are supported under ESSA? The authors then describe the ESSA-defined tiers of evidence that such funding streams will require. They compare ESSA evidence tiers with evidence requirements for other federal education programs to identify ambiguities in the ESSA tiers. The key question for this section is: How are the ESSA evidence tiers defined, how does current guidance clarify these tiers, and what further guidance might improve the use of these evidence tiers for education decisionmaking? Next, they provide a brief description of their methodology in reviewing the literature and present findings on improvement activities that could reasonably be interpreted to fit within ESSA's evidence framework, given the statute and guidance. Finally, recommendations are offered to guide education policymakers, practitioners, and thought leaders on the use of research-based practices. A bibliography is included. [This report expands on the version released in January 2017 ("School Leadership Interventions Under the Every Student Succeeds Act: Evidence Review--Updated and Expanded," by Rebecca Herman, Susan M. Gates, Emilio R. Chavez-Herrerias, and Mark Harris) by including information about the interventions reviewed in Appendix D. For earlier (2016) versions of this report, see ED571819 and ED570943.]