This study examined the role of outcome expectancies in explaining sex differences in adolescents' inclination to engage in acts of aggression. Using a sample of Australian adolescents, 2 scales were developed to measure outcome expectancies of direct and indirect aggression. The data were used in subsequent mediation analyses. Sex differences in direct aggression were partially mediated by 2 types of aggression expectancies. Relative to females, males anticipated more overall benefits associated with acting aggressively and less fear of reprisals from authority figures. This pattern of expectancy beliefs predicted higher levels of self-reported direct aggression. Sex differences in indirect aggression were fully mediated by anticipation of personal benefits, with males expecting greater personal benefits for engaging in this type of aggression.