Although many studies have examined the predictors of overall substance use among adolescents, few have focused on the high school setting as a specific context for substance use. Therefore, predictors of alcohol and marijuana use at school were examined in a sample of high school students. The general hypotheses were that substance use at school depends on (a) personal predispositions, (b) the situational opportunity for substance use at school, and (c) the interaction of predispositions and opportunity. Several interactions were found suggesting that personal predispositions are more strongly related to substance use at school when students believe they have the opportunity to use alcohol and drugs without getting caught.