Adolescence is a period of heightened experimentation with risky behavior. Models of brain development suggest that this phenomenon is partly the result of increased adolescent sensation seeking unaccompanied by maturation in ability to evaluate risks. We test an alternative biosocial-affect model in which favorable affect attached to behavior leads to discounting of risks. Although the model applies to both adolescents and adults, it predicts that the surge in sensation seeking during adolescence increases affective attraction to risky behavior, reduces perceived risk of the behavior, and results in peer-group reinforcement of these effects. We estimated models that included these influences for three drugs (tobacco, alcohol, and marijuana) in a national sample of youth ages 14 to 22. Consistent with brain maturation models, sensation seeking rose during the age period under study with girls peaking earlier than boys. Nevertheless, independent of age or gender, the biosocial-affect model explained the relation between sensation seeking and drug use. The findings indicate that although adolescents recognize the risks of drug use, they are subject to both biological and social influences that encourage risk taking. Implications for the prevention of risky adolescent behavior are discussed.