School leaders across the United States face the challenge of creating safe and secure environments across their campuses in a way that helps promote the achievement of schools' educational objectives. To help them meet this challenge, the U.S. Department of Homeland Security's Cybersecurity and Infrastructure Security Agency asked the Homeland Security Operational Analysis Center to conduct a review of the literature on physical security in kindergarten-through-12th grade (K-12) schools and other, comparable sectors to improve school leaders' understanding of how system-based security operations and physical elements can protect school occupants and mitigate the impact of threats and risks. In this report, the authors define and present a systems approach to school physical security in which five protection and mitigation elements--security equipment and technology, site and building design, people and personnel, policies and procedures, and training and exercises--integrate and work together to provide layered security benefits. Specifically, the approach to protection and mitigation elaborated here emphasizes that the policies, procedures, and training developed around school physical security tie together people and personnel with equipment, technology, and design choices to build a coherent system. School leaders need to think about how the protection and mitigation options they have in place affect the broader school mission, operations, and climate. Schools with diversity in student bodies, campus locations, and other traits will approach physical security in unique ways that match their unique needs, recognizing that there is no one-size-fits-all solution to protection and mitigation. [For the companion report, "Challenges in Implementing Physical Security Measures in K-12 Schools. Research Report. RR-A1077-2," see ED615799.]