Basic processes of socialization by parents and peers involving modeling and social reinforcement, in their direct, indirect, and contingent manifestations, are investigated with respect to adolescent use of alcohol and marijuana. Four causal models are tested for frequency of alcohol and of marijuana use and for initiation into alcohol and into marijuana. The analyses are based on cross-sectional and short-term longitudinal samples of adolescent-parent-best schoolfriend triads. The basic socialization processes are found to vary depending upon the source of influence, the specific drug, and the phases of behavior engaged in by the adolescent. Implications of the results for socialization theory are discussed.