As the development of relations between international regional organizations, interregionalism denotes a relatively recent phenomenon. Largely due to systemic bipolarity, inter-regional relations remained limited to ‘dialogue partnerships’ between the European Community (EC) and other regional groupings until the end of the Cold War. Academic interest in the topic is even more recent, and until 2001 the terminology of inter-regionalism was rarely used. 1 Scholars have, moreover, mainly focused on inter-regionalism involving the European Union (EU). While acknowledging the key role of the EU, we also include other, less studied inter-regional relations. Views differ as to what is covered by the term ‘inter-regionalism’. We distinguish between regionalization , which refers to the growth of socio-economic integration within a given region, and regionalism , which is the establishment of regional organizations resulting from the top-down political response of states to bottom-up processes of regionalization. Regional organizations are formal institutions, created by international treaties. Based on this distinction, inter-regionalism is the process and outcome of political and economic interactions between regional organizations. We distinguish between ‘pure’ inter-regionalism, involving two regional organizations, and ‘hybrid’ inter-regionalism, involving regional organizations and other regional actors as well. Our defi nition is not limited to ‘inter-hemispheric cooperation’. It also includes relations between regional organizations on the same continent. In fact, a continent is composed of different regions, the relations between which constitute an inter-regional layer of governance. Pioneer of inter-regionalism Heiner Hänggi also includes so-called ‘trans-regional’ relations between groups of states, such as the Asia Pacifi c Economic Cooperation (APEC), composed of 15 East Asian, two North American and two South American countries (Hänggi 2006). However, we agree with Julie Gilson that trans-regionalism and inter-regionalism are different phenomena.