This chapter assesses the general concept of security and the way in which issues come to be ‘securitized’. The security of the sovereign state, in a system of states, and existing under conditions of anarchy, has been the traditional focus of studies in global or international politics. Security in this context has therefore been concerned largely with the threats that states pose to each other. Over the last few decades, however, the agenda for security in global politics has expanded, and so too has its conceptualization. The chapter looks at traditional approaches to security and insecurity, revisiting the Hobbesian state of nature and tracing security thinking in global politics through to the end of the Cold War. This is followed by a discussion of ideas about collective security as embodied in the UN, paying particular attention to the role of the Security Council and the issue of intervention in the post-Cold War period. This period has also seen the broadening of the security agenda to encompass concerns such as gender security, environmental security, cyber security, and the diffuse concept of ‘human security’. Finally, the chapter provides an overview of the ‘war on terror’, raising further questions concerning how best to deal with non-conventional security threats.