This contribution investigates the concept of rule of law in the discourse of EU law: When is the concept used? What does it mean? What does it allow doing? The expression first appears in the Treaty of Maastricht and is classically used to affirm a value that is common to the EU and to the Member States. In the current context of crisis of the rule of law, the debate mainly deals with the enforcement of the rule of law by the Member States: scholars typically highlight the weaknesses of article 7 TEU and seek alternative enforcement mechanisms. In the Court’s case law, the concept has been used to state that the EU is “a Union based on the rule of law”. In some famous cases, this sentence has served to justify the recognition of new remedies and procedures for the Court to review the legality of EU measures. Quite paradoxically, in other cases, it has also served to justify the affirmation that this recognition was not necessary. Interestingly, in a series of rulings in 2008, the Court established a connection between the enforcement of the rule of law by the Member States and the affirmation of a Union based on the rule of law. This case law has led to a Europeanization of the concept of rule of law, which goes hand in hand with a process of judicialization: national and European courts acquire a central role in supervising the respect of rule of law by the Member States.