International organizations are ubiquitous in contemporary Europe and the wider world. This special issue takes a historical approach to exploring their relations with each other in Western Europe between 1967 and 1992. The authors seek to 'provincialize' and 'de-centre' the European Union's role, exploring the interactions of its predecessors with other organizations like NATO, the OECD and the Council of Europe. This article develops the new historical-research agenda of co-operation and competition among IOs and their role in European co-operation. The first section discusses the limited existing work on such questions among historians and in adjacent disciplines. The second section introduces the five articles and their main arguments. The third section goes on to elaborate common findings, especially regarding what the authors call the vectors for the development of policy ideas and practices and their transfer across different institutional platforms.