Addiction to various substances, including drugs and alcohol, probably arises from a combination of environmental and genetic factors. The genetic vulnerability to drug addiction is supported by several familial, adoption and twin studies. However, as in other mental disorders, the genetic vulnerability to drug addiction appears complex: these disorders do not follow the rules of Mendelian inheritance. Instead, they are probably influenced by multiple susceptibility genes, each of which contributes to the disorder. The more genes necessary for a disorder, the harder it is to detect any of them. This difficulty is magnified by the role of environmental factors. Association studies using the candidate gene approach can identify susceptibility genes for drug abuse supported by the pathophysiological hypothesis of the illness. This review will focus on the clinical and molecular genetic studies in drug abuse.