Differentiated integration in the European Union – the differential validity of EU rules across territories – receives substantial interest from both academia and the public. While primary law differentiation is increasingly well understood, secondary law differentiation remains largely uninvestigated. The extent of opt-outs of member states from EU policies as adopted in directives and regulations is, except for a few prominent cases, largely unknown. This paper is based on a new and comprehensive dataset on secondary law differentiation collected at the ETH Zurich and the University of Konstanz. Its aim is to establish both descriptive and analytical insights into the topic, focusing on the extent of differentiation across types of legal act, decision-making procedures, and policy-areas. The study’s results allow for new conclusions about the character of European integration and the European Union. While this paper does not directly relate to the discussion about federalism in the European Union, it adds a dimension to the debate by highlighting the extent to which EU policies are applied uniformly across the Union in the first place.
