Policymakers and practitioners alike have increasingly positioned instructional materials as essential for achieving high-quality, standards-aligned instruction. State education agencies, school districts, and schools have looked to instructional materials to signal what to teach and to what depth for students to be on track to meet college- and career-readiness standards. As instructional leaders, principals play a critical mediation role in communicating district curriculum policies to teachers and supporting teachers' enactment of curriculum. However, very little work has explored how principals take on this role and what school leaders need most to engage effectively with issues and decisions regarding instructional materials. This brief brings together key findings from three RAND Education & Labor reports on principals' roles with respect to instructional materials. The findings point to several recommendations for how various stakeholders--federal and state policymakers, principal preparation programs, districts, and curriculum and professional learning developers--can better prepare and support principals to lead on the selection and use of instructional materials. [For the first report, "How Instructional Materials Are Used and Supported in U.S. K-12 Classrooms: Findings from the 2019 American Instructional Resources Survey," see ED614109. For the second report, "Teachers' Perceptions of What Makes Instructional Materials Engaging, Appropriately Challenging, and Usable: A Survey and Interview Study of Teachers," see ED614115. For the third report, "School Leaders' Role in Selecting and Supporting Teachers' Use of Instructional Materials: An Interview Study," see ED614075.]