Like many other common, complex disorders, alcoholism tends to run in families. Evidence from animal studies as well as biochemical, electrophysiologic, and personality studies of alcoholics and their offspring indicate the existence of heritable factors predisposing to the development of alcoholism. Twin and adoption studies also support the idea that vulnerability to alcoholism depends in part on genetic factors. The observed inheritance, however, does not fit any simple pattern, indicating a substantial degree of etiologic heterogeneity in the alcoholism phenotype, with the ultimate manifestation of the disorder dependent on poorly understood gene-environment interactions. Efforts to address these issues, including the hunt for biological markers, defining more homogeneous subtypes of alcoholism, and molecular genetic strategies of potential use in alcoholism are reviewed.