Peer influence is widely believed to be major cause of cigarette, marijuana, and alcohol use in adolescents. The design of more effective intervention programs to decrease adolescent substance use would be enhanced by a better understanding of how peer influence is transmitted. This study is designed to look at both the process of peer selection and peer influence in 6th through 12th graders. The design is a longitudinal sequence in which 6th, 8th, and 10th graders will be followed for two years. The relative influence of four aspects of the peer social network on adolescent substance use will be examined. These four aspects are best friend dyads, clique leaders, the clique as a whole, and the social crowd. Analyses will be done to determine whether sex, age, type of substance used, or stage of use affects the source or the amount of peer influence. The longitudinal design will allow influence to be examined relatively unconfounded by selective association. To examine the determinants of peer selection unconfounded by the effects of peer influence, friendships that exist at the later time points of the study, but not at the early ones, will be examined for variables in which similarity predates the friendship. Age and sex effects in the determinants of friendship choice will be examined. A model of peer influence on adolescent cigarette smoking, which suggests that normative pressure is much more important than direct pressure form peers in leading to smoking will be tested on this sample. The model will be improved by adding multiple indicators of direct pressure and will be tested for marijuana, and alcohol use as well as cigarette smoking. Finally, the study results will be used to propose a model of peer influence that includes both peer selection and peer influence.