Since the end of the Cold War, the notion of 'security' has significantly changed and has thus impacted security studies in particular as well as the broader field of IR in general. Whereas security was traditionally defined in terms of military capacity and the management of armed conflict between sovereign states, the post-Cold War era identified many new threats that would potentially challenge international security. Hence, the conduct of international security is constantly changing, being faced with previously unaddressed issues and approaches. 'Security' itself remains a deeply contested concept. What is (international) security? What characterizes the shift in international security and what are its impacts on the future? Can international security be achieved? In order to answer these questions, we will establish the different levels of security, look into the ongoing academic debates, and discuss some of the key concepts that shape contemporary security studies.