This paper outlines a new approach to the study of bystander intervention. Using insights derived from self-categorization theory (Turner, Hogg, Oakes, Reicher, & Wetherell, 1987), we explore the social category relations among those present in the context of physical violence. The paper describes two experiments that manipulate the social category relations between (a) bystander and fellow bystanders, and (b) bystander and victim. Analysis indicates that fellow bystanders are only influential when they are in-group rather than out-group members. Furthermore, bystanders are more likely to help victims who are described as in-group as opposed to out-group members. Overall, the findings suggest an important role for a self-categorization perspective in developing strategies to promote bystander intervention.