Drug addiction is a serious behavioral disorder. It is a long term evolving process characterized by dependency. It is not only biological and psychological but also social. It should be differentiated from occasional drug use, from drug abuse with minor consequences as well from medical drug dependency in which the individual does not seek a psychological effect. Drug addiction causes are multiple and include psychological, family, social and obviously biological factors (drug addicting potential). In to-day clinical practice, addiction to benzodiazepines or opioids prescribed for maintenance therapy and the newly described secondary addiction of later onset are predominant. Severe social consequences and many psychiatric and medical disorders in particular due to HIV infection also result from drug addiction. Depressive and psychotic disorders cause specific therapeutic problems when patients deny their troubles or when drug use produces special relief. These psychological disorders frequently underlie medical care needs.