This chapter examines the definitions of regionalism (the political and economic cooperation among states within a defined geographical area) and regionalization (the process of social, economic, and cultural interaction through which regions evolve and integrate), as well as related key concepts. Defining the borders of regions is a complex yet necessary process, as they shape regional identities and governance structures. Modern regionalism since 1945 is characterized by the voluntary association of states, driven by the needs of society and the economy. The book highlights four types of regional entities: top-down structures, such as the EU; bottom-up, supply-chain-based cases, like East Asia; regional free trade areas, like EFTA; and regions without regionalism, like the Middle East. Regionalism is dynamic, influenced by historical relationships, social constructs, and varying geopolitical mentalities. Drivers of regionalism include economic interdependence, security concerns, and shared cultural identities. The interplay between cooperation and conflict, as well as the roles of institutions and identity, is vital for understanding contemporary regional dynamics, which are complex and ever-changing. Today, regional cooperation and integration evolve amidst the challenges posed by deglobalization and rising nationalism.