The concept of primary international institutions is a core idea of the English School and central to those scholars from Bull to Buzan who have sought to take it in a more sociological direction. Yet the English School has traditionally found it difficult to define and identify with consistency the institutions of international society. A group of scholars, here called the 'new institutionalists', have recently sought to address this problem by devising tighter definitions and applying them more rigorously. But different understandings and lists of institutions continue to proliferate. The source of the problem is the reliance on 'stipulative' definitions, drawn from an increasingly abstract theoretical literature. The problem is compounded by the new institutionalists' employment of social structural and other 'outsider' methods of social research. This article argues that it is only possible to empirically ground institutions, a task on which all agree, by returning to the interpretive 'insider' approach traditionally associated with the school-but employing it in a much more rigorous way. To this end it makes the case for a 'grounded theory' of international institutions inspired by Chicago School sociology.The English School (ES) concept of primary international institutions, having lain dormant for several decades, has recently been picked up by a number of theorists (e.g. Buzan, 2004;Holsti, 2004;Navari, 2011;Schouenborg, 2011;Clark, 2011) and deemed a highly promising tool for empirical analysis and theoretical development in International Relations (IR). Primary institutions, to provide a preliminary definition, are not formal organizations such as the EU, NATO or the UN, but 'set[s] of habits and practices shaped towards the realization of common goals' (Bull, 1977: 74). They are more diffuse and fundamental than international institutions in the 'organizations', and in ES parlance, 'secondary' sense. When it is claimed (e.g. Suganami, 1983; Suganami, 2003: 253-4; Buzan, 2004: 161, 166; Dunne, 2001a: 69-80) that 2 concern for the institutional structure of inter-state relations is a defining feature of the ES, or that 'Bull … worked in conjunction with a group of theorists and practitioners, now identified as the founding fathers of the English school of international relations, who stressed the importance of the institutional foundations of state behaviour ' (Little and Williams, 2006: 1), it is institutions in the primary sense to which reference is being made. According to Buzan (2005b: 190) the concept of primary institutions is the 'core idea' of the ES. In Clark 's view (2011: 46) it is its 'central insight'. According to Schouenborg (2011: 27) it is its 'central concept'.Given this it is perhaps surprising that there is little agreement within the ES about the number and identity of international society's primary institutions. Various lists have been drawn up with some containing as few as three institutions and one containing as many as 26. Initially the problem was considered to b...