Is the process of European integration shifting the targets of social movements from the national arena to the European Union level? Does the EU remain isolated from the reach of national democratic politics? Or are social movements responding to the transnational issues posed by the European Union, but continuing to do so by treading the traditional pathways of nation-specific contentious politics? Addressing these key but hitherto unexplored questions, this groundbreaking volume explores how European citizens are responding to the growing policymaking power of the EU. The contributors argue that the emergence of a supranational realm of European government offers new opportunities and constraints for domestic social actors. In this new transnational realm, groups such as environmentalists, students, and trade unionists can not only undertake traditional domestic forms of contentious action, but also link together across national borders. At the same time, the volume cautions against rushing to herald a new era of transnational mobilization, as significant barriers remain to launching contentious action in the transnational realm, national governments continue to play a primary role in policymaking before the EU, and tried-and-true routines of collective action and institutions attach citizens to their national political systems. At the heart of the book is a newly developed theoretical framework, which is rigorously tested against the evidence offered by the editors' longitudinal and cross-national database on contentious political action, as well as by a series of sector-specific case studies. The combination of theory and original research will make this an important reference for scholars as well as a valuable supplement in courses on comparative politics, Western Europe, and social movements.