While an association between antisocial personality disorder (APD) and substance use disorder (SUD) has been frequently observed, the causes of the comorbidity remain unclear. Adoption and twin studies have found evidence of both genetic and environmental influences in APD and SUD. Therefore, comorbidity between APD and SUD may be the result of shared genetic influences, shared environmental influences, or a combination of the two. However, only a limited number of adoption and twin studies have addressed this issue and the results have not been conclusive. In future studies, a distinction should be made between alcohol and drug abuse and between juvenile and adult APD symptoms. Twin samples of adequate size would allow use of structural equation analytical methods for estimation of the relative magnitude of genetic and environmental influences shared between the two conditions, as well as influences contributing to each specifically. Results would be highly relevant for the clinical setting as well as for efforts to identify the genes involved in either trait.