This chapter presents an overview of the problems of enlargement for the European Union (EU) and the accession countries, followed by a sketch of the main theoretical approaches used to explain enlargement and Europeanization. It offers an analysis of the two facets of enlargement policy based on evidence from the 2004/07 eastern enlargement process, the accession negotiations with Turkey, and the current policy towards the Western Balkans. The analysis of EU enlargement and non-member Europeanization largely follows the theoretical lead of institutional theories in International Relations, most prominently the debate between 'rationalism' and 'constructivism', two families of theories developed to explain the causes and effects of international institutions. Ankara Agreement contains vague references to membership, the Contracting Parties examine the possibility of the accession of Turkey to the Community, as soon as the economic association and the customs union advanced far enough. The chapter finally draws theoretical conclusions and discusses the limits of EU enlargement.