In this chapter, students will learn about the complex relationships between culture and security. Culture is present in organizations, societies, states and global affairs. Culture, like the social world in general, is ubiquitous. The chapter therefore begins by summarizing why security scholars began to think that culture might shape processes of global and national security. It then clarifies what we mean by culture and how we think it matters. Contrasting rational and cultural approaches to security, the chapter examines several important issues: (1) the importance of bureaucratic and organizational culture for understanding security issues such as military doctrine and evolution; (2) strategic culture; and (3) how organizational and global cultural factors influence peace operations.