In this chapter, we investigate the activities of scholar-practitioners in the trading zones between theory and practice of international relations. These trading zones are the place where ‘ideologies of international organisation’ take shape, understood as recognisable and recurring patterns of justificatory arguments that define the scope and competences of an international organisation, relating them to a set of political goals and values depicted as universal and largely uncontroversial. Drawing on the seminal work of Robert W. Cox, we are particularly interested in the way in which such ideologies serve the interests of international organisations and their exponents. Scholar-practitioners were in a prominent position to shape common understandings of what international organisations were and what they were good for. They addressed diverse audiences including not only civil servants and member state representatives but also donors, civil society groups and not least the scholarly community in the field of political sciences and law. We illustrate our theoretical argument by introducing three scholar-practitioners from the early and mid-twentieth century who tried to anchor social-democratic values and strategies in international organisations: James T. Shotwell, David Mitrany and C. Wilfred Jenks.