Although early use of alcohol during adolescence has been consistently associated with increased risk of alcoholism in adulthood, the specific mechanisms that underlie this association remain unclear. We describe a program of epidemiological twin-family research that shows that early use of alcohol is best conceptualized as an indicator of a more general propensity to engage in adolescent problem behavior. Adolescent problem behavior, in turn, is a risk factor for a broad range of adult externalizing disorders, of which alcoholism is but one manifestation. These findings are shown to be consistent with a dual-process model whereby early adolescent problem behavior is associated with increased risk of adult psychopathology because both are indicators of a common inherited liability and because early adolescent problem behavior increases the likelihood an adolescent is exposed to high-risk environments. We conclude with a discussion of the importance of cross-cultural research, which may be especially informative for identifying the consequences of early adolescent drinking.