Affiliation: Professor of Government at Cornell University (Sgt2@cornell.edu). His research interests span three fields: social movements and parties, local and regional politics, and European and transnational politics. His most recent single-authored book is Power in Movement. With collaborators Charles Tilly and Doug McAdam, he has written Dynamics of Contention, and with Doug Imig, Contentious Europeans. For comments on earlier versions of this essay, Tarrow thanks Antonina Gentile, Charles Tilly, Frances Fox Piven, and two anonymous reviewers.