The pervasive use of drugs in the general population and the clearly established relationship between drug abuse and psychological disorder emphasizes the importance of diagnoses and treatment in both the mental health and the drug abuse service systems. Therapeutic community (TC) research findings on psychopathology are highlighted on the prevalence, trends, and impact of treatment, which indicate that TCs have evolved a potent model for modifying drug abuse behavior and reducing psychological symptoms. Although its success has been most evident with the primary substance abuser, the TC has much to offer the increasing numbers of dually diagnosed individuals who manifest their dysfunctionality both through drug abuse and psychological maladjustment.