Enlargement has always been an essential part of the European integration. Each enlargement round has left its mark on the integration project. However, it was the expansion of the European Union (EU) with the 10 Central and Eastern European Countries (CEECs), Cyprus, and Malta, unprecedented in scope and scale, which presented the EU with an opportunity to develop a multifaceted set of instruments and transformed enlargement into one of the EU’s most successful policies. The numerous challenges of the accession process, along with the immensity of the historical mission to unify Europe, lent speed to the emergence of the study of EU enlargement as a key research area. The early studies investigated the puzzle of the EU’s decision to enlarge with the CEECs, and the costs and benefits of the Eastern expansion. However, the questions about the impact of EU enlargement policy inspired a new research agenda. Studies of the influence of the EU on candidate and potential candidate countries have not only widened the research focus of Europeanization studies (beyond the member states of the Union), but also stimulated and shaped the debates on the scope and effectiveness of EU conditionality. Most of the analytical frameworks developed in the context of the Eastern enlargement have favored rational institutionalist approaches highlighting a credible membership perspective as the key explanatory variable. However, studies analyzing the impact of enlargement policy on the Western Balkan countries and Turkey have shed light on some of the limitations of the rationalist approaches and sought to identify new explanatory factors.
After the completion of the fifth enlargement with the accession of Bulgaria and Romania in 2007, the research shifted to analyzing the continuity and change of EU enlargement policy and its impact on the candidate and potential candidate countries. There is also a growing number of studies examining the sustainability of the impact of EU conditionality after accession by looking into new members’ compliance with EU rules. The impact of EU enlargement policy on the development of European Neighbourhood Policy (ENP) and comparative evaluations of the Union’s performance across the two policy frameworks have also shaped and expanded the debate on the mechanisms and effectiveness of the EU’s influence. The impact of the Eastern enlargement on EU institutions and policymaking is another area of research that has emerged over the last decade. In less than two decades, the study of EU enlargement policy has produced a rich and diverse body of literature that has shaped the broader research agendas on Europeanization, implementation, and compliance and EU policymaking. Comprehensive theoretical and empirical studies have allowed us to develop a detailed understanding of the impact of the EU on the political and economic transformations in Central and Eastern Europe. The ongoing accession process provides more opportunities to study the evolving nature of EU enlargement policy, its impact on candidate countries, the development of EU policies, and the advancement of the integration project.