This chapter provides an overview of concepts, theoretical perspectives, and research findings important to the discussion of revolutionary social movements. A social movement can be defined as a persistent, organized effort by a relatively large number of people either to bring about social change or to resist it. Some examples of the many social movements in the history of the United States include the antislavery movement, antiwar movements, the antipoverty movement, the civil rights movement, the women's rights movement, and the occupy movement. Sociologists and other social scientists have often attempted to classify revolutions into one of two ideal types: leftist or rightist. Just as social movements in general are difficult to categorize as either totally liberal or totally conservative, many revolutions include both leftist and rightist characteristics. In comparison, the "revolutions through democracy" of the twenty-first century in Venezuela and Bolivia were generally viewed as primarily leftist in orientation.