Owing to the vagaries of the ‘constitutionalization’ of the European Union, legal scholarship has disregarded the momentous constitutional transformation brought about by the European Convention System. This is regrettable, not least because the Convention has reconfigured national constitutional authority in a cosmopolitan context. The emerging cosmopolitan constitutionalism is based upon three ideas: first, the exercise of state authority must also be legitimate from the perspective of those who are not citizens; second, a constitution must embrace fundamental rights and the representation of insiders in order to facilitate the representation of all, including outsiders; third, the authority of the constitution doesn’t just depend on endorsement by an independent people but also on recognition by other peoples who pursue the same type of political project. At the same time, any cosmopolitan constitutional system needs to leave space for particularity. It is therefore not accidental that the idea of a ‘margin of appreciation’ is of pivotal significance.