The co-occurrence of substance abuse and psychiatric disorders is a complex and common phenomenon. The National Institute of Mental Health Epidemiologic Catchment Area (ECA) study and the National Comorbidity Study (NCS) are two large epidemiological surveys that have evaluated the prevalence of comorbid psychiatric and substance use disorders in community samples. In the ECA study, 45% of individuals with alcohol use disorders had at least one co-occurring psychiatric disorder (Regier et al. 1990). Likewise, in the NCS, 78% of alcohol-dependent men and 86% of alcohol-dependent women met lifetime criteria for another psychiatric disorder, including drug dependence (Kessler et al. 1996). The co-occurrence of psychiatric and alcohol use disorders is clinically important because comorbidity has a negative impact on the course, treatment outcome, and prognosis of both syndromes.