This article explores the epidemiology of co-occurring disorders (CODs) with an emphasis on the implications of study findings for the functioning and potential of substance abuse treatment. Severity of disorder is discussed as an issue that may have particular significance for the selection of specialized as opposed to traditional substance abuse treatment forms. Exploration is made, as well, of the resources currently available to substance abuse treatment, especially the human resources available, and the implications of resource availability for undertaking initiatives specific to COD. Findings from standard and enhanced treatment for comorbid individuals are examined in an effort to clarify areas of need for specialized and typical treatment personnel. Issues are raised for consideration by the clinical research and treatment provider communities in terms of assessment and diagnosis, manpower and training, and response to the challenge of relapse in this population.