Psychiatric co-morbidity among substance users refers to the simultaneous presence of at least another psychiatric disorder in a person diagnosed with a substance use disorder. Co-morbid patients represent a substantial number of people in treatment and present greater disorder severity from both the clinical and social perspectives than those people diagnosed with only one type of disorder. We present an overview of the current state of the art concerning the choice of site of treatment, the kind of intervention, the length of such treatment, and future goals, aiming to establish a more effective intervention, and finally so as to further improve clinical outcomes.