This study analyzed peer-influence versus peer-selection mechanisms in adolescent tobacco, alcohol, and marijuana use. Participants were surveyed 3 times, with 1-year intervals, about peers' substance use and their own use; Sample 1 had 1,190 participants (initial mean age = 12.4 years), Sample 2 had 1,277 participants (initial mean age = 11.5 years). Latent growth analyses that were based on composite scores indicated that initial peer use was positively related to rate of change in adolescent use, supporting the influence mechanism; there was little evidence for a selection mechanism. Difficult temperament, poor self-control, and deviance-prone attitudes were related to initial levels for both peer and adolescent use. It is concluded that peer influence is the primary mechanism during middle adolescence. Temperament-related attributes may be predisposing to early experimentation and deviant-peer affiliations.