Résumé
The questions relating to the temporal applicability of the European Union law arise as soon as the European Union adopts a new act. It is systematically necessary to determine the instant from which the act products its effects and the facts to which it applies. Furthermore, the principle of uniformity of the European Union law means that these questions must be answered by the European Union law itself. In spite of their importance these questions have been neglected by the European Union legal literature. Yet the case-law of the European Union Court of Justice and the acts adopted by the European Union (primary law, secondary legislation and international agreements) provide a wealth of information. Indeed, the Court of Justice has established principles which constitute the general framework of the temporal applicability of the European Union law. However relevant, this general framework must be completed by transitionnal provisions, a set of written rules adopted on a case-by-case basis. These transitional provisions, at first deeply questionable due to being hard to identify and interpret, turn out to be a critical tool for the temporal applicability of the European Union law.