Until the late 1980s, European integration was generally said to be based on ‘integration through law’. However, in the 1990s, a group of scholars observed that law seemed to be increasingly challenged and new modes of governance tended to distance themselves from legal integration. They were confronted with another group arguing that the EU was still subject to processes of judicialisation, juridification and Eurolegalism. Against this background, this special issue aims at understanding what triggers normative transformations, be it the hardening or the softening of law, at both EU and national levels. In this introduction, soft and hard law are given a definition based on obligation and enforcement. Then, normative transformations are situated in a framework which makes a distinction between processes, normative levels and political/administrative levels of transformation. Finally, three types of explanations are suggested to help us understand when and why transformations occur.