The theory's primary learning mechanism is operant conditioning, in which behavior is shaped by the stimuli which follow or are consequences of the behavior. Social behavior is acquired both through direct conditioning and through imitation or modeling of other's behavior. Behavior is strengthened through reward and avoidance of punishment or weakened by aversive stimuli and loss of reward. The major explanatory variables from this theory -- differential association, differential reinforcement, definitions, and imitation -- combine to account for 68 percent of the variance in adolescents' marijuana use and 55 percent of the variance in their alcohol use. The most powerful variable is differential association, although the other variables stand on their own in explaining substantial portions of variance. Empirical support for this theory suggests that it will will have utility in explaining adolescent use and abuse of other substances. Data tables, over 70 references, and a list of social learning variables are included.