Research in psychology, public health, and sociology has identified personality characteristics (e.g., depression, hostility, sensation-seeking) and characteristics of the social environment (e.g., peer and family influences, socioeconomic factors, stressful life events) as predictors of adolescents use of tobacco, alcohol, and other drugs. However, psychosocial research typically has not considered biological and genetic risk and protective factors as possible sources of resilience to substance use. Simultaneously, behavioral geneticists have investigated the etiology of substance use behavior by partitioning its phenotypic variance into genetic and environmental components, including shared and nonshared environments: Unfortunately, much of this research has been limited by imprecise measures of environmental risk and protective factors. This study will bridge the disciplines of psychology, sociology, public health, and behavioral genetics by using the methods of multiple disciplines to evaluate a model of adolescent tobacco and alcohol use etiology. We will collect detailed data about tobacco and alcohol use, social-environmental factors, and personality characteristics among 600 pairs of adolescent twins in Southern California and 600 pairs of adolescent twins in Qingdao, China. This study will distinguish environmental risk and protective factors from heritable factors and identify the specific aspects of the shared and nonshared environments that are associated with substance use. This study also will assess interactions between heritable factors and the social environment in the prediction of adolescent tobacco and alcohol use, to determine whether heritable characteristics make some adolescents more susceptible or resilient to risk factors in the social environment.