The school of thought known as Realism (with a large R) has been a central focus of debate in international theory. Nevertheless, its content and epistemological status (and therefore the criteria for its evaluation) remain elusive. In part this is due to the variety of contexts and debates within which Realism has been discussed in the field. In the 1930s and 1940s the debate was framed around a Realist-Idealist axis. In the 1970s Realism was contested by liberal analyses of the causes and consequences of an allegedly growing global interdependence. In the 1980s there emerged a three-cornered debate between competing Kuhnian ‘paradigms’, among which Realism dominated. Given that the meaning of Realism has been partly constructed by historically variable theoretical and political issues, its identity has also varied over time. If, however, one chooses to think about Realism outside of these various contexts within which it has been both defined and evaluated, it remains unclear how such diverse thinkers as Thucydides, Machiavelli, Hobbes, Carr, Morgenthau and Kenneth Waltz can coherently be considered as part of a single tradition of thought. Sensitivity to their differences may be a virtue for the historian of ideas, but it runs the risk of undermining the point of the exercise! Despite these problems, which are only partially corrected by distinguishing between types of Realism (notably classical and ‘neo’ or ‘structural’ varieties), most scholars would agree with Alan James that Realism is a school whose members harbour shared assumptions about the primacy of states as international actors, the separation of domestic and international politics, and who describe the latter in terms of anarchy and a concomitant ubiquitous struggle for power and security.