Four elements are common in social science definitions of social movements: a network structure, the use of unconventional means, shared beliefs and solidarity, and the pursuit of some conflictual aims. Once a marginal area in the social sciences, social movement studies grew into a main field of study in sociology and a significant one in other proximate disciplines, such as political science, anthropology, geography, history, and psychology. This growth was accompanied by significant shifts in the ways in which social movements have been addressed. Before the 1970s, especially (but not only) in the United States, social movements were conceived of as forms of collective behavior, different from “normal” behavior because of high emotionality and, often, anomic syndromes; in Europe instead, especially the main historical social movement, the labor movement, was addressed within a Marxist perspective, with attention paid to the structural conditions for its development. Since the 1970s, both approaches were shaken by the spread of new forms of protest. First of all, in the United States, studies on the civil rights movement showed that it was neither irrational nor anomic, being instead guided by strategic behavior and strong normative systems. Social movement organizations started therefore to be seen as actors that mobilize resources in their environment for collective action. In Europe, the emergence of the student, the women’s, and the environmental movements were considered as examples of new social movements, bound to substitute for the increasingly institutionalized labor movement. Class-based approaches were therefore supplanted by an attention to emerging collective identities. Social movement studies in the 1970s and the following decade focused mainly on macro-level political opportunities for protest and organizational forms and strategies at the meso level, with only limited attention to social structures and individual commitment. Since the 1990s, this structural bias was then challenged by a renewed attention to various cultural aspects, as well as to the causal mechanisms that intervene between structure and action in a field redefined as contentious politics and covering social movements as well as revolutions, democratization, and other contentious phenomena.