Building on the theory of reasoned action (I. Ajzen & M. Fishbein, 1973, 1980; M. Fishbein & I. Ajzen, 1975) and expectancy theory, the authors examined the mediating role of alcohol expectancies in adolescent drinking behaviors by testing whether alcohol expectancy outcomes and valuations (the extent to which these outcomes are perceived as good or bad) mediate the association between peer influences and lifetime alcohol use. Early adolescents (N = 904) from 2 public middle schools in western Michigan completed a battery of questionnaires. Overall, results showed that alcohol expectancies and valuations partially mediated the relations between peer influences (peer use and peer approval) and lifetime alcohol use. The findings suggest that associating with peers who are perceived as using alcohol and approving of drinking may influence adolescents' alcohol expectancies. The authors briefly discuss future research directions and implications for prevention. (Contains 2 tables and 1 figure.)