This article calls for a re-conceptualization of the social sciences by asking for a cosmopolitan turn. The intellectual undertaking of redefining cosmopolitanism is a trans-disciplinary one, which includes geography, anthropology, ethnology, international relations, international law, political philosophy and political theory, and now sociology and social theory. Methodological nationalism, which subsumes society under the nation-state, has until now made this task almost impossible. The alternative, a ‘cosmopolitan outlook’, is a contested term and project. Cosmopolitanism must not be equalized with the global (or globalization), with ‘world system theory’ (Wallerstein), with ‘world polity’ (Meyer and others), or with ‘world-society’ (Luhmann). All of those concepts presuppose basic dualisms, such as domestic/foreign or national/international, which in reality have become ambiguous. Methodological cosmopolitanism opens up new horizons by demonstrating how we can make the empirical investigation of border crossings and other transnational phenomena possible.