<p>This article summarizes the findings of research articles examining the consequences of the eastward enlargement of the European Union after 20 years. Experiences from the processes of adjustment to the EU’s governance regime prior to accession have been interpreted as a success story as the Central and Eastern European new member states moved from the EU periphery “back to the heart of Europe”. However, assumptions about the EU’s transformative power on the new members after accession are rather ambiguous. This is partly due to the inability of the EU to enforce the rules once the candidate country became a member, but also due to the fact that the countries were ready to join a completely different Union, which at that time was not yet characterized by economic crises, migration, security threats, or centrifugal forces leading to disintegration. This thematic issue aims to contribute to a better understanding of the processes underlying the EU’s changing political order by answering the following questions: To what extent, why, and how have Central and Eastern European countries contributed to the transformation of the EU political system?</p>