This study explores aggression among siblings. Specifically, this study tests whether different types of aggression among siblings occur more or less frequently than toward friends and acquaintances. This study also examines whether sex differences in aggression between siblings resemble sex differences in aggression among friends and acquaintances. Results suggest that direct aggression occurs much more frequently among siblings than among friends and acquaintances, but that indirect aggression occurs far less frequently among siblings than among friends and acquaintances. In addition, the findings suggest that certain sex differences in aggression, such as males directly aggressing more often than females, are diminished when the aggression occurs between siblings. Both males and females report high levels of direct aggression toward their siblings.