Recent increases in domestic and international acts of extremist violence perpetrated against American citizens have prompted an increased need for information to help understand and evaluate the threat posed to U.S. targets by extremist groups and their individual members. The purpose of this paper is to (i) suggest the potential relevance of social psychological research on group behavior for understanding and assessing threats of extremist group violence; and, (ii) encourage more systematic research on group violence to further inform assessments of group risk. Approaching the issue from the levels of group behavior, and of individual behavior within a group context, the article summarizes research on key principles of group behavior, and the effects of group membership on individual behavior; proposes specific questions derived from these principles for consideration in evaluating risk for violence by groups, and by individuals influenced by groups; and suggests further research needs.