Many studies have suggested the importance of peer influence and personal attitudes (e.g., expectancies, resistance self-efficacy, and perceived harm) in predicting adolescent use of illegal substances. The present study examined these variables in relation to self-reported use of alcohol, cigarettes, and marijuana for 213 younger adolescents (12-15 years old) and 219 older adolescents (18-22 years old). A series of logistic regressions was performed to assess variables relating to use of each substance by age group and gender. Friends' use was significantly related to substance use for both age groups, both sexes, and all substances examined in this study. Perceived harm was not significantly related to use for any group. Finally, outcome expectancies and resistance self-efficacy were differentially related to use depending on age, gender, and substance. The implications of these findings for prevention programming and future research are also discussed.