Sumário:The historical roots of constitutionalism -- European constitutionalism : a second revolution -- Relationship to national constitutions -- What is the European Union?: The citizen's perspective. A union of law (Walter Hallstein. Three basic principles of the UE -- The relationship in the light of multilevel constitutionalism: Multilevel constitutionalism and European constitutionalism. European Verfassungsverbund : unity in diversity. The principles of mutual respect and cooperation -- Brexit in the light of European constitutionalism: The perspective of the citizens. Procedural safeguards for withdrawal. Sovereignty regained - or lost?.