Sibling violence is the most prevalent and least studied form of family violence, and little research has examined differences based on severity. This research examines more severe versus less severe forms of sibling violence. Using a subsample of married couples with two or more children ages 0 to 17 drawn from the 1976 National Survey of Physical Violence in American Families, the authors employ Conflict Tactics Scale items for child-to-child conflict to construct a measure of sibling violence severity. Drawing from several theoretical perspectives on family violence and peer aggression, the authors analyze the impact of macro-system variables, family stress and resources, and family subsystems on less severe and more severe sibling violence. Contextual factors are most important in explaining less severe sibling violence. Experience of parental violence and unpredictability are individual factors relevant to severe sibling violence. More research is needed to examine the etiology and impact of different forms of sibling violence. (Contains 4 tables.)