This chapter explores the concept of region, defined as a territorial entity distinct from both locality and nation-state. The region constitutes an economic, political, administrative, and/or cultural space, within which different types of human agency interact, and towards which individuals and communities may develop attachments and identities. Regionalism is the manifestation of values, attitudes, opinions, preferences, claims, behaviours, interests, attachments, and identities that can be associated with a particular region. The chapter first reviews the main theories and approaches that are used to understand the political role and importance of regions, including the modernization paradigm, Marxism, and institutionalism. It then considers the various dimensions and aspects of regions and regionalism, with particular emphasis on regionalism from below versus regionalization ‘from above’. It also examines the political economy of regions, tracing the changing economic role and place of regions within the national and global economy.