This chapter examines the pervasiveness and importance of enlargement in the history of European integration. It first considers the principles, conditions, and instruments of enlargement before discussing the roles of various institutional actors and the candidate states. It then shows how, faced with the likelihood of large-scale Central and Eastern European accession, the European Union extended the requirements for membership to include the candidate countries' democratic credentials and economic competitiveness. The first enlargement included Britain, Denmark, and Ireland, followed by Greece, Spain, and Portugal, the European Free Trade Association, the Central and Eastern European countries, Cyprus, and Malta. The chapter also explains how the EU has developed a variety of strategies to deal with growing differences among the member states' socio-economic situations and policy needs without formally resorting to a division of its membership in concentric circles, core and peripheral groups, or alternative frameworks.