Power is one of the most commonly used concepts in the social sciences and in ordinary political discourse. It would seem to be central to the study of international relations. Before we consider the role of power in international relations in the short twentieth century, it is necessary to clear up some preliminaries about the concept in general. This is because power is a frustrating concept; its use seldom lives up to our expectations. To begin with there is a fundamental ambiguity; is power a phenomenon to be explained or an explanatory tool, a dependent or an independent variable? On closer inspection, the explanatory power of the concept of power is low. It is difficult to explain social relations and their outcomes, at either the national or the international level, in terms of power as it is normally conceived in the social sciences. Yet, however theoretically sophisticated we try to be, we are driven to use some such concept for want of an alternative.