Long a focal point in the study of Geography, regions have become a major concern of International Relations, and for some even its essence. Principle definitions and approaches, however, remain contested, as do the contexts in which and how they matter, from economic to security. This article examines contested views on what constitutes a region and on the nature and functioning of regional architecture, drawing from thematic and case-specific literature to indicate the expanse of analytical enquiry. These include the roles and interpretations of geography, identity, culture, institutionalisation, and the role of actors, including a hegemon, major regional powers and others actors from within a region, both state and societal. A final section indicates additional areas for future research.