The process of enlargement has transformed the European Union. It has had far-reaching implications for the shape and definition of Europe, and for the institutional set-up and the major policies of the Union. This has been accomplished through a number of enlargement rounds, which the first section of the chapter analyses in detail. This is followed by a review of the enlargement process itself, with a focus on the use of conditionality and the role of the main actors involved. The contributions of neo-functionalism, liberal intergovernmentalism, and social constructivism to explaining the EU’s geographical expansion are evaluated in the third section of the chapter. The success and prospect of future enlargement are discussed in the context of wider EU developments, especially the effect of the economic crisis in the euro area, ‘enlargement fatigue’, the domestic context in the candidate countries, and Brexit.