The term “security” is as ambiguous in content as in format: is it a goal, an issue-area, a concept, a research program, or a discipline? There is no one concept of security; “national security,” “international security,” and “global security” refer to different sets of issues and have their origins in different historical and philosophical contexts. The author argues that the concept of international security might most appropriately describe current security affairs. She challenges the concept of national security as fixated on the nation-state and not taking into account the security of other states. She criticizes the notion of global security as presupposing a world-wide common definition of security and shared sets of values, rules, and principles not yet existing. In the long term, however, the world might be moving in the direction of a global security system if institution-building continues and leads to common practices, rules, and enforcement capabilities. As all concepts yield only limited explanations and are of marginal value for theory-building, the essay identifies some assumptions and questions to be clarified in future research programs. In a closing section the field of international security studies and its relationship to international relations are discussed.