This book challenges the conventional wisdom that the European Union (EU) is an example of governance without opposition. Building on a concept of opposition that honors both the institutional features of the EU polity and the fundamental functions of political opposition, it argues that legal mobilization and litigation before constitutional courts provides actors and organizations from civil society with an opportunity to challenge and overturn policy decisions that originate at the EU level. Further, it presents case studies of constitutional challenges to the implementation of the EU Data Retention Directive in four Member States (Austria, the Czech Republic, Germany and Ireland) to illustrate the motivations for, as well as the preconditions and pitfalls of, mobilizing constitutional law. By connecting the literature on social movements, law and politics and comparative government, this book will appeal to readers interested in political science, sociology and legal studies.