Enrique Lara¿a is Titular Professor of Sociology at the University of Madrid, Spain. Hank Johnston is a lecturer in the Department of Sociology at San Diego State University. Joseph R. Gusfield is Professor Emeritus of Sociology at the University of California, San Diego. Contributors: Jos¿lvarez-Junco, Robert Benford, Richard Flacks, Scott Hunt, Bert Klandermans, Alberto Melucci, Doug McAdam, John McCarthy, Carol Mueller, M¿ Szab¿, David A. Snow, Ralph H. Turner, and the editors.