Social control refers to any reaction by which a bystander communicates to the “perpetrator” of an uncivil behavior that his or her action is not acceptable. In 3 field studies, we examined the factors that affect people's tendency to exert social control. Passersby in the streets were asked how they would react were they to witness different uncivil behaviors. They also rated their appraisal of the situation and the emotions they would feel. The results suggest that 3 factors are primary determinants of social control: the feeling of responsibility to exert it; the perceived legitimacy of social control in the situation; and the extent to which bystanders felt hostile emotions. These results have implications for how to reduce uncivil behaviors.