This article is a revised version of the 2006 Martin Wight Memorial Lecture and examines the placeof regional states-systems or regional international societies within understandings of contemporary international society as whole. It addresses the relationship between the one world and the many worlds-on one side, the one world of globalizing capitalism, of global security dynamics, of a global political system that, for many, revolves a single hegemonic power, of global institutions and global governance, and of the drive to develop and embed a global cosmopolitan ethic; and, on the other side, the extent to which regions and the regional level of practice and of analysis havebecome more firmly established as important elements of the architecture of world politics; and the extent to which a multiregional system of international relations may be emerging. The first section considers explanations of the place of regionalism in contemporary international society and the various ways in which the one world aff ects the many. The second section deals with how regionalism might best be studied. The final section analyses four ways in which regionalism may contribute to international order and global governance.