This module provides an overview of theoretical perspectives on security, broadly defined. Drawing on classic and contemporary literature from International Relations and other academic disciplines, we consider the nature of security as a state-of-being or socio-political practice, and we inquire into a series of fundamental questions: security of what? Security for whom? Against what? Over what time period? By what means? At what cost? Throughout the module, attention focuses on what security means in theory and practice, what it could become, and what security should be about and why. The module is informed by ongoing research in the Department of Politics and International Relations (PAIR), and it complements other modules that explore global governance, global ethics, foreign policy, and military strategy. For students enrolled in the MSc International Security and Risk degree, the theory-driven approach in Security Theory complements the issue-based approach in Contemporary Security Challenges (PAIR6002).