At the intersection of the history of constitutional ideas and of political theory, this book offers a new genealogy of the constitutional thought of the European Union. Centrally, the book traces the emergence and transformation of the ‘post-sovereign thesis’ – an argument that seeks to move beyond the routine opposition between states and European organization, by claiming the concept of sovereignty to be obsolete – and its complicated relationship with liberalism. Analyzing the thought of a series of constitutional thinkers who have developed different versions of this thesis in relation to European integration, the book shows that, far from being new, as is generally assumed, the post-sovereign thesis goes back to the late nineteenth century. Exploring the interplay of these thinkers’ critical conceptualizations of sovereignty and of their views on liberalism, the book argues that, although they shared a concern for the transformation of a world seen as increasingly interdependent, they imagined deeply different versions of post-sovereignty. Bringing this history into focus, the book offers a rich new perspective on contemporary debates about the EU and the possibilities of global constitutionalism. This book will appeal to scholars and students working in the fields of EU and constitutional law, legal history and the history of political thought, as well as others with relevant interests working in political science.