Twenty-five years ago, theoretical reflection on International Relations (IR) was dominated by three broad discourses. In the United States the behavioural revolution of the 1950s and 1960s had helped to create a field that was heavily influenced by various assumptions allegedly derived from the natural sciences. Of course, variety existed within the behaviourist camp. Some preferred the heavily quantitative approach that had become especially influential in the 1960s, while others were exploring the burgeoning literature of rational and public choice, derived from the game theoretic approaches pioneered at the RAND corporation. Perhaps the most influential theoretical voice of the late 1970s, Kenneth Waltz, chose neither; instead he developed his Theory of International Politics around an austere conception of parsimony and systems derived from his reading in contemporary philosophy of science.