Identifying crucial deficiencies in the legal theories of Hart and Dworkin, The Rule of Reason in European Constitutionalism and Citizenshipdraws on the work of Habermas and Alexy in order to elaborate a new juridical conception of citizenship. Taking Europe as its focus, the central substantive argument of the book is that a 'rule of reason' governs supranational constitutionalism. Addressing the free movement law of the internal market, and recently developed case law on the free movement of citizens, it shows how the contours of European and member state legal systems, as well as European and national citizenship, are delimited through an ongoing judicial discourse. The constant drawing and re-drawing of the boundaries of member state and European law is negotiated in a judicial conversation that, it is argued, constitutes a key characteristic of supranational constitutionalism: one that supports a thin, juridical, and essentially rights based, conceptualisation of European citizenship. Offering an innovative theoretical analysis of EU law, The Rule of Reason in European Constitutionalism and Citizenship will be of considerable interest to scholars of European law, European politics and legal theory.