The vast IR literature on security has for a long time neglected defining the concept of security, relying on an almost axiomatic identification between security amd military affairs. However, in the international context born out of the end of the Cold War, the recognition of an increasing link between internal and external events and the increasing perception of security threats of a non-military nature, have been widely recognized both by governments and IR scholars. This has resulted in the widening of the security agenda and in a debate on the viability of the traditional definition of security. After describing the evolution of the concept of security, the article analyzes the conceptualization of security and the contribution of Buzan and the "Copenhagen school" to the theoretical debate on the redefinition of the concept of security.