This book offers a comprehensive analysis of the new framework of relationship between the United Kingdom (UK) and the European Union (EU) applicable since 1st January 2021, following the end of the Brexit transition period and the entry into force of the EU-UK Trade & Cooperation Agreement (TCA), concluded on Christmas Eve 2020. The book contextualizes the new framework of EU-UK relations, including the ongoing challenges of implementing the Withdrawal Agreement (WA), and sheds light on the new mechanisms for EU-UK cooperation both in the economic domain including free movement of goods, financial services, and mobility of persons, and in the security domain including law enforcement, defence, and data protection. The work underlines the profound differences between the new status quo compared to the legal framework applicable when the UK was still an EU member state including end of free movement of persons, financial passporting, and cooperation in foreign affairs and defence, and reflects on what the latest stage in the Brexit process means for governance, sovereignty, and the future of European integration.