This chapter addresses two issues: What are social movements and How and why do they develop. It defines social movements more precisely and describes various types of movements. The chapter discusses various perspectives on the origins and causes of social movements. Most sociological studies of movements have focused on the second issue. The chapter examines more concretely how social reform movements have transformed American society. The chapter focuses on social revolutions and illustrates them with an extended discussion of the collapse of communism that began to unfold in Eastern Europe and the Soviet Union in the late 1980s, the Iranian Revolution of 1979, and the ongoing revolution referred to as the Arab Spring. It describes several varieties of individual explanations, micro-social explanations, and macrostructural explanations. Individual explanations of social movements focus on the psychological characteristics of individuals.