Why do states leave regional organisations – and what happens next? This book explores these timely questions through the analytical lens of comparative regionalism. While Brexit may be the most high-profile example of a state departure, this book shows that state withdrawals from regional organisations are neither unique nor isolated. From West Africa to the Andes, from the Pacific Islands to Eurasia, state exits have become a recurring albeit limited feature of the regional landscape challenging the future contours of the international order.This book offers a comparative analysis of voluntary withdrawals from regional organisations. Anchored in the conceptual framework of differentiated disintegration, the authors examine such exits not as abrupt ends, but as forms of institutional change within the broader tapestry of regionalism. The result is a multidimensional portrait of how regional cooperation evolves – and occasionally unravels.The volume begins with a robust conceptual and methodological foundation, situating withdrawals within broader debates on regionalism, integration, and institutional development. It draws on diverse integration theories and bridges the divide between European and Global South experiences. The authors take the reader across five continents with seven richly documented case studies – including the UK’s departure from the EU, Morocco’s exit from the Organisation of African Unity (OAU), Chile and the Andean Community, and Kiribati leaving the Pacific Islands Forum. Each chapter follows a common analytical structure, enabling comparative insight into drivers, consequences, and post-withdrawal re-engagements, that is differentiated disintegration.