The role of peers in adolescent drug use is analyzed by integrating differential association and situational group pressure notions with propositions derived from social bond theory. The resultant causal model is tested for self-reported use of alcohol, cigarettes, marijuana, amphetamines, and depressants. The model explains 49% of the variation in combined drug use among 768 adolescents. The variables from social learning traditions have the strongest effects in the model, but associations with drug-using parents or with prodrug definitions received from parents or friends have far less impact on drug use than do associations with drug-using friends per se. It is argued that differential association with situational pressure to join others in drug use may be more relevant to adolescent drug use than are social bonds or differential association with a ratio of prodrug to antidrug definitions.