The European Union, as a regulatory polity based on integration through law, arguably relies more on legal compliance with its policies than any other political system. Proceeding from this point of departure, this book puts the spotlight on the subnational tier and scrutinizes its role in ensuring compliance. Drawing on a dataset of infringement proceedings against federal and regionalized member states, the book shows that strong shared rule, i.e., strong cooperation between national and subnational authorities, can improve national compliance records. In contrast, policy sectors with strong redistributive consequences impair subnational authorities’ capacity to comply.  In short, policy and politics matter more than polity.