In 1970, the Foreign Ministers of the European Economic Community’s Member States began meeting in a new informal structure called European Political Co-operation (EPC). EPC followed a mode of policy-making known as the ‘intergovernmental method’, a mode with substantial differences to the more orthodox supranational mode of policy-making used in the European Economic Community (EEC). After EPC, intergovernmental policy-making was established in new areas of collaboration including fiscal policy, defence policy, and internal security. What differentiated the intergovernmental method was its emphasis on the co-ordination of national policies over the emphasis on compromise, package deals, and collective policy-making epitomised by supranational governance. This was achieved using national vetoes, allowing Member States to veto costly new policies, avoid compliance where common policies became too costly, and limit supranational agencies in their ability to cultivate further integration. Intergovernmental policy-making thus allowed Member States to protect their national interests in sensitive areas of policy closely related to state sovereignty. After nearly fifty years of intergovernmental policy-making in Europe, these distinctive characteristics of the intergovernmental method have changed; decision-making procedures emphasising collective decision-making have become more common, there has been an increase in the formalisation and legalisation of structures and decisions, and an increase in the use of supranational agencies. This thesis labels this process of institutional change as ‘communitarisation’ and asks; how and why do the EU Member States move intergovernmental policy-making processes, emphasising the co-ordination of national policies, towards more communitarised (though not strictly supranational) structures emphasising co-operation through common policies? In answering this, the thesis has three main aims. Firstly, it argues that the intergovernmental method should be understood as existing not as an alternate mode of policy-making but as part of a wider process of integration called communitarisation. Secondly, it claims that communitarisation is a general phenomenon found across differing sensitive areas of policies and offers a generalised conceptualisation of this phenomenon. Thirdly, it contends that communitarisation is primarily driven by Member State interests through a process of bargaining, reflecting the bargaining power of Member States and their attempts to balance the costs of interdependence with the potential adjustment costs associated with integration.