Problem and pathological gambling (PG) occurs in about 5% of Americans. Gambling is associated with substantial psychosocial and psychiatric health problems, and the increasing ease of access to gambling may increase its future prevalence. Therefore, it is important to gain greater insight into the causes of PG. Family studies of PG are consistent with a substantial familial impact on vulnerability to PG. However, family studies cannot distinguish genetic from family environmental influences. By contrast, the study of twin pairs permits the genetic and environmental influences on PG to be estimated. The study of gambling behavior among 3,359 twin pair members of the Vietnam Era Twin Registry suggests that: (1) inherited factors explain a substantial proportion of the variance in the report of symptoms of gambling; (2) there is a single continuum of genetic vulnerability that underlies gambling problems of varying severities; and, (3) the co-occurrence of PG with conduct disorder, antisocial personality disorder, and alcohol abuse/dependence is partially explained by genes that influence both PG and these other psychiatric disorders. Neurophysiological correlates of gambling problems and genetically based differences in neurotransmitter systems may provide biological mechanisms that explain the genetic basis for a predisposition to PG.