Recent major epidemiological surveys in general population samples throughout the world have demonstrated that substance use disorders are among the most frequent forms of mental disorders in the community, and are also frequently associated with other forms of mental disorders. This paper briefly reviews the concept of comorbidity and summarizes more recent data concerning the frequency of comorbidity of substance use disorders. The review is limited to studies in the general population using standardized diagnostic interviews. Specific emphasis is laid upon the most recent data from the National Comorbidity Survey, that specifically addressed comorbidity issues in detail. The clear majority of subjects with a definite substance use disorder according to the strict DSM-III-R definition has or has had at least one other comorbid mental disorder. Comorbidity rates between specific substance use disorders and other mental disorders will be compared and discussed in light of several other international epidemiological studies. Furthermore time sequences of substance use disorders and comorbid disorders are presented. Potential pathogenetic and clinical implications are addressed.