This chapter discusses the development of a social movement. It considers the movement organization and some influences on how it operates. The chapter reviews early theories of collective behavior, which attributed many of these behaviors to emotional processes in crowds. It also reviews the social scientific evidence about riots, collected over several decades. A social movement is collective activity that expresses a high level of concern about some issue. Contemporary events that appear to reflect contagion involve a number of people in close proximity displaying the same behavior or symptoms of illness, with no apparent physical cause. Emergent norm theory applies to collective behavior that occurs when people find themselves in an undefined or unanticipated situation. The strain model emphasizes the individual's emotional state in explaining collective behavior. Gatherings have a purpose; participants' behaviors reflect that purpose in interaction with features of the setting.