Many histories have been told of the study of international relations, some starting as long ago as classical Athens. This chapter is interested, however, in International Relations (IR) as one of the institutionally differentiated disciplines that together make up the contemporary social sciences. IR arose early in the twentieth century as one sub-field in an increasingly modernist political science. Scholars of IR have followed to a considerable extent the modernist trends that have dominated that wider discipline. Of course, there have been times when scholars of international relations have given their own twist to modernist political science and even times when they have followed their own fashions. Generally, however, much is gained by locating the history of IR against the background of modernism in political science. We begin this chapter, therefore, with an analysis of the modernism and positivism that shaped the study of political science and international relations during much of the twentieth century. In the second half, we examine more recent trends in IR. American IR has been characterized by growing polarization between, on the one side, positivists and rational choice theorists, and on the other side, a broad methodological modernism including institutionalism and constructivism. In Britain, a broad methodological modernism dominated during this period, often in the form of “reluctant modernist” approaches, and despite a late “normative turn”. Methodological Modernism, ca. 1918–1950 The history of twentieth-century IR is, for the most part, a story of scholars adopting and adapting, or reacting against, the rising tide of modernism in political science. Modernism emerged in the early twentieth century to replace the developmental historicism that had earlier dominated the study of politics. Modernism cut across earlier historical narratives of continuity, dividing the political world into discrete and discontinuous units. It made sense of these units through analytic schemes and sometimes impersonal mathematical rules. It used ahistorical typologies and calculations to frame narratives and sometimes to replace narrative explanations altogether. One should not imagine, however, that modernism was either homogenous or hegemonic. On the contrary, modernism arose alongside shifts in the topics and techniques favoured by political scientists, and although these trends tended over time to mutually reinforce one another, each had its own roots, and participation in one did not necessitate participation in the other.