This chapter examines the claim that realism offers the most powerful explanation for the state of war that is the regular condition of life in the international system. It first provides an overview of the theory of realism before discussing whether there is one realism or a variety of realisms. It argues that despite some important differences, all realist theories share a set of core assumptions and ideas. It goes on to consider these common elements, namely self-help, statism, and survival. The chapter concludes with an analysis of the extent to which realism is relevant for understanding the globalization of world politics. To illustrate the main ideas tackled in this chapter, two case studies are presented: one relating to the Melian dialogue and the other to strategic partnerships with ‘friendly’ dictators. There is also an Opposing Opinions box that asks whether U.S. hegemony is durable or fleeting.