This open access book, the eighth volume in the series of Interdisciplinary European Studies, explores the implications of the EU’s size and depth at a time of war in Europe. Russia’s fateful decision to invade Ukraine in 2022 has put the question of EU enlargement back on the political agenda and, again, the implications of a larger and more diverse EU bring key principles of integration to the fore. Addressing these issues, scholars from economics, law, and political science provide insights from previous EU enlargements and the UK’s withdrawal from the EU. Based on up-to-date research findings, they provide succinct assessments of the challenges facing the EU in areas such as immigration, labour market integration, and adaptation to advanced technologies. To conclude, each chapter offers policy recommendations for decision-makers on European and national levels.