The concept of integration has long stood at the heart of scholarly and policy debates on migration, citizenship, and social cohesion. Yet, its meanings, uses, and implications remain deeply contested. This chapter explores how integration has been conceptualised across historical, sociological, and political traditions, tracing its evolution from classical sociological theory to contemporary critiques and reformulations. By examining integration through different lenses, including liberal egalitarianism, multiculturalism, civic integrationism, and transnationalism, the chapter aims to provide a comprehensive overview of how integration has been defined, studied, and measured. It also critically assesses the normative assumptions embedded in integration research and policy, particularly the tendency to individualise and decontextualise integration, often reinforcing majority norms. The chapter concludes by highlighting recent theoretical shifts toward relational and boundary-based approaches, which offer more dynamic and inclusive ways of understanding integration in an increasingly fluid and diverse world.