The idea of the critical mass is central to many understandings of collective action. Social movement activists, community leaders, and fund-raisers use the term when they talk about getting together enough resources to accomplish some goal. They express the understanding that it takes some minimum number of people or some minimum accumulation of seed money to draw in the participation and contributions of others. The phenomenon can also be seen in other, less intentional forms of collective action. Lynchings, wildcat strikes, and riots proceed when people become convinced that enough others are participating. Detailed observational studies of spontaneous collective action show incipient “organizers” calling to others to join their action, and they show potential participants talking to each other and reaching a common agreement about who will act (see McPhail 1971; Berk 1974; Fantasia 1988). The riot was the archetypal phenomenon of interest for early analysts of collective action.