Catalonia and Scotland are home to two of the most well-known nationalist movements in Western Europe. In the last few years, debates on autonomy and independence have dominated political discussions in both territories, putting pressure on political elites in Spain and the UK to articulate a positive vision of political partnership and avoid state disintegration. The recent growth ofpro-independence support in Catalonia and Scotland poses an evidentchallenge to the continuation of the Spanish and UK states, but as this book makes clear, so too does the re-emergence of an increasingly intolerant right-wing Spanish nationalism and inflexible British unionism. Nations in flux compares the historical and contemporary trajectories and political dynamics of Catalan and Scottish nationalism. Drawing on a rich collection of interviews with politicians, policy documents and existing research, this book explores the development of territorial politics in both cases since 2010. It analyses the experiences of dramatic moments such as the 2014 Scottish independence referendum, the UK's withdrawal from the European Union, the 2017 Catalan independence referendum and the onslaught of the COVID-19 pandemic and argues that existing approaches to territorial accommodation are inadequate and require reform. Building on these findings, and through the lens of multinational federalism, it reimagines the design and operation of the Catalan and Scottish territorial models. Offering in-depth analysis and a detailed comparison of the Catalan and Scottish cases, this book highlights the increasing relevance of nationalism in contemporary society and makes a significant contribution to the scholarship of territorial politics.