This article shows how widespread and how volatile the language of constitutionalism has become in today's EU. It poses the baseline question of the very possibility of a constitutional law for the EU — a question that all positions in favour of a constitution, written or unwritten, are bound to answer affirmatively. The article begins by considering the EU against a general background of constitutional imagination and definition. In so doing, it explains why our understanding of the EU is influenced by the historic centrality of the modern state to constitutional theory and practice, but also why, in these inescapable but incomplete terms, the EU is an unresolved constitutional entity. It then considers how the EU's putatively constitutional features have emerged and unfolded, in so doing focusing on the centrality of law. And as this centrality has come under pressure in the mature EU, the article looks at the changing constitutional challenges and opportunities of this new post-state polity.