The study of regions in international relations has been a sometime thing, gaining scholarly attention in the 1950s and 1960s, dropping largely from view in the 1970s, and returning to focus quite dramatically in the late 1980s and early 1990s. It is clear that the contemporary manifestations of regionalism (the completion of the internal market of the European Communities in 1992, Asia Pacific developments, and US-centred Western hemisphere moves) constitute a new and qualitatively different factor in both interstate relations and the international political economy. The growth in the development of regions in the 1980s also represents a new level of interstate collaboration in the international system. The question arises as to the causes of this 'new regionalism' of the 1980s, and the implications of these developments for international relations practice and theory. Investigating these issues is the main task of this thesis. ¶ This thesis involves three elements: a central contemporary element examining the re-emergence of regions in the 1980s; a second comparative element comparing the causal factors operating in three different regions; and lastly, a theoretical element examining the usefulness of current theory to the phenomenon of regionalism in the 1980s and 1990s. Chapters Two and Three discuss the relevant theoretical literature with a view to developing the propositions to be examined in the case studies. They examine three of the major streams of international relations theory - realism, liberal economics, and institutionalism - with a focus on what these contending theories have had to say about how regional groupings arise. Chapter Two looks at the relevant theoretical literature in the 1950s and 1960s while Chapter Three explores the more recent theoretical literature of the 1970s and 1980s. ¶ The rest of the body of the thesis tests propositions set out at the end of Chapter Three on the causes of the regionalist revival in the 1980s by way of three case studies, each one concerned with the actual development of regionalism in three different parts of the globe: Western Europe, North America and the Asia Pacific. ¶ In all three regions the move towards regionalism was clearly a reaction to negative developments in the international economic and political systems. It was in part a specific response to the undermining of the liberal international trading regime and the associated rise in protectionism, particularly in the US. It was also partly the result of an ideational shift in terms of economic doctrine away from keynesianism and import substitution industrialisation to economic liberalism and export-oriented economic growth. Yet it is also apparent from the case studies that the new regionalism was also to some extent the result of a kind of interactive chain reaction, a spiral of mutual anxiety, with regionalism in one area provoking an extension of regionalism in another. It is indeed difficult to establish which of these causal explanations is the principal one as it is clear from the case studies that they are in fact mutually reinforcing. ¶ The thesis concludes with an analysis of the insights provided by the case studies into the theoretical debates examined in Chapters Two and Three. Finally, there is an attempt to use these insights to construct a theory accounting for the rise of the new regionalism.