Alcoholism is a complex disorder involving both genetic and environmental factors and interactions between them. Localizing and characterizing the genetic influences on susceptibility to alcohol dependence may provide new insights into pathology and new avenues for treatment and prevention. However, because of the complex nature of the disorder, the binary categorization of individuals as affected or unaffected may be a poor indicator of their underlying genetic susceptibility. Quantitative risk factors, or endophenotypes, that differentiate levels of severity among affected individuals and levels of susceptibility among unaffected individuals, provide one solution to this problem. Genetic studies of such quantitative risk factors in families of probands with alcohol dependence may help to disentangle the complex genetic architecture of this disorder.