So stark and swift was the collapse of the Cold War structure of international relations that few yet pretend to have been expecting it. The magnitude of the changes involved has forced practitioners and theorists alike into radical rethinking. For practitioners, the old certainties have gone and it is unclear what political and security structures will replace those of the Cold War: whether it will be a New World Order or a New World Disorder is still very much open to debate. But for international relations theorists the events have focused attention on the nature of international political structures. What kind of structures can international systems represent if they can be changed so fundamentally and so easily? Neo-realists especially have to rethink a dominant discourse which relies heavily on established regularities and on the stability of the bipolar system. What does it say for Waltz's conception of international structure if it can be so easily transcended by unit factors? If structural theories of international relations can say nothing about an event as momentous as the collapse of the Cold War system, what can they say anything about? Neo-realists could ignore the fact that their theories could not account for transformations of international structure precisely because these theories did explain the regularity and stability of bipolarity. Now that is gone, theorists have to look again at what they mean by a structure. Moreover, the nature of agency has to be reexamined; for neo-realists human agency was essentially irrelevant at the structural level of explanation, yet the collapse of the Cold War system seemed to depend very largely on active and calculating agents. Questions concerning the nature of agency and the meaning of structure and the relationship between them are now more relevant than ever in international relations theory.