International relations has often been treated as a separate discipline distinct from the other major fields in political science, namely American and comparative politics. A main reason for this distinction has been the claim that politics in the international system is radically different from politics domestically. The degree of divergence between international relations (IR) and the rest of political science has waxed and waned over the years; however, in the past decade it seems to have lessened. This process has occurred mainly in the “rationalist research paradigm,” and there it has both substantive and methodological components. Scholars in this paradigm have increasingly appreciated that politics in the international realm is not so different from that internal to states, and vice versa. This rationalist institutionalist research agenda thus challenges two of the main assumptions in IR theory. Moreover, scholars across the three fields now tend to employ the same methods. The last decade has seen increasing cross-fertilization of the fields around the importance of institutional analysis. Such analysis implies a particular concern with the mechanisms of collective choice in situations of strategic interaction. Some of the new tools in American and comparative politics allow the complex, strategic interactions among domestic and international agents to be understood in a more systematic and cumulative way.