In the UK, Brexit has led to a new public and scholarly debate about the constitutional implications of membership in the EU. Leading UK public law scholars now argue that membership has entailed a constitutional transformation to the extent that the old doctrine of ‘parliamentary sovereignty’ no longer rings true. EU membership has paved the way for judicial review, the adoption of fundamentals rights, a distinction between ‘constitutional statutes’ and ordinary legislation, and introduced the idea that ‘popular sovereignty’ can express itself via referenda. What is less discussed is that the process of constitutional transformation is not unique to the UK, but has taken place, albeit in different ways, in all the Member States. This paper discusses three different manifestations of constitutional transformation and the implications for (popular) sovereignty: (1) the ‘evolutionary democracies’ (2) the ‘post-fascist’ member states and (3) the ‘post-communist’ member states.