Psychiatric comorbidity is defined as the co-occurrence of a psychiatric disorder in a patient with a substance use disorder. Psychiatric disorders in substance abuse patients can antedate the substance use disorder or be a consequence of the substance abuse. There is emerging evidence that drug use in adolescence may alter the onset of certain psychiatric disorders in vulnerable individuals. Patients with concurrent comorbid disorders present special challenges for the substance abuse treatment system in terms of diagnosis and management because each disorder has the capability of exacerbating the other. This manuscript is a summary of an ISAM symposium that featured three speakers who discussed the following topics: 1. Etiology and treatment of comorbid psychiatric and substance use disorders in adolescents; 2. Treatment of ADHD and substance use disorders in adults; 3. Effects of substance abuse on the onset, severity, and treatment of schizophrenia. Recommendations for further research will be presented.