The Routledge Handbook of Differentiation in the European Union offers an essential collection of ground-breaking chapters reflecting on the causes and consequences of this complex phenomenon.

Benjamin Leruth is an Assistant Professor in European Politics and Society at the University of Groningen, The Netherlands.

Stefan Gänzle is Professor of Political Science and Head of the Department of Political Science and Management and a member of the Jean Monnet Centre of Excellence, University of Agder (UiA), Norway.

Jarle Trondal is a Professor at University of Agder, Department of Political Science and Management, and a Professor at University of Oslo, ARENA Centre for European Studies, Norway.

1. Introduction Part 1: Conceptualising differentiation Introduction 2. Studying differentiated integration: Methods and Data 3. On the legitimacy of differentiated integration 4. Differentiated (dis)integration beyond Europe: A Comparative Regionalism Approach 5. De facto Differentiation in the European Union: Circumventing Rules, Law, and Rule of Law 6. Constitutive differentiation 7. EU External Differentiated Integration and Compliance: Theoretical and Legal Aspects 8. From Integration to Fragmegration: Political symbols and the emergence of differentiated European identities 9. Differentiation and segmentation Part 2: Institutionally-based differentiation Introduction 10. An ever more fragmented Union? On the emerging relevance of differentiated integration for governance structures within the EU 11. Promise Unfulfilled? Managing Differentiated Integration in EU Secondary Law Through Enhanced Cooperation 12. Differentiation in the European Parliament: United about diversity? 13. Differentiation and the European Central Bank: A bulwark against (differentiated) disintegration? 14. Differentiation and the European Commission 15. Differentiation and the European Court of Justice 16. Third country participation in EU agencies: Towards "condominio"? 17. The Council of the European Union: Organizational and Social Dynamics of Differentiation Part 3: Policy-based differentiation Introduction 18. Differentiated integration in EU energy market policy 19. Brexit and the Common Fisheries Policy: Opportunities for multi-level differentiated (dis)integration? 20. The European Asylum Policy: Core state powers, flexibility and differentiated integration 21. Differentiated integration in EU climate policy 22. Differentiated integration in European External Action 23. PESCO: A formula for positive integration in European defence 24. The Increasingly Differentiated European Single Market? 25. Differentiation and social policy: A sustainable way forward? 26. Differentiation in EU Security and Defense Policy Part 4: Territorial differentiation Introduction 27. The Nordic countries as pioneers of differentiation 28. Risky Advantageous Differentiation: Iceland and the EEA 29. The Swiss and Liechtenstein relations with the EU - an ongoing institutional challenge 30. Turkey's external differentiated integration with the EU in the field of migration governance: the case of border management 31. Visegrad Four and EU Differentiated Integration: Activities, Perception and Self-Perception after the Refugee Crisis 32. Poland as the (new) awkward partner: Differentiated integration or differentiated disintegration? 33. Trajectories of Differentiated EU Integration for the Western Balkans 34. European Neighbourhood Policy: Differentiated integration beyond the EU's Eastern and Southern Borders 35. Differentiation at the local level: An overview of sub-national authority networks in the EU Part 5: Brexit Introduction 36. Brexit as a phenomenon: national solidarity as a tool against the European project? 37. (Post-)Brexit: Negotiating Differentiated Disintegration 38. International perceptions of Brexit 39. Differentiation and Power Asymmetry: How Brexit is Changing UK Relations with Czechia and Slovakia 40. Brexit and Northern Ireland 41. Border Conflicts and Territorial Differentiation after Brexit: The cases of Northern Ireland, Gibraltar and the UK Sovereign Base Areas in Cyprus 42. Growing Apart Together? Brexit and the Dynamics of Differentiated Disintegration in Security and Defense 43. Conclusion. Epilogue: Polycrisis and Resilience in the European Union: Covid-19 and avenues for future studies