Realism is widely regarded as the most influential theoretical tradition in International Relations, even by its harshest critics. Its ancient philosophical heritage, its powerful and original critique of liberal internationalism, together with its influence on the practice of international diplomacy have secured it an important, if no longer dominant position in the discipline. No other theory has given as much form and structure to the study of international politics, especially to the sub-fields of Security Studies and International Political Economy (see Donnelly 2000).