Although substance use disorders (SUDs) occur commonly in the psychiatrically ill, they are often underrecognized and undertreated. This has dire consequences for both individual patients and society in general. The immense economic burden of alcohol- and drug-related disorders alone and their associated adverse consequences has been estimated to cost the United States $375 billion annually (Office of National Drug Control Policy 2001). Moreover, in comparison with patients with only a mental illness or an SUD, patients with co-occurring mental illness and an SUD across a broad spectrum of diagnostic types and combinations have greater severity of illness and a worse longitudinal course of illness in multiple domains, including increased risk for psychiatric and substance use relapses; higher rates of recidivism; higher levels of psychological distress; poorer psychosocial functioning; worse treatment retention; poor medication compliance; higher rates of violence, suicide, legal difficulties, medical problems, and family stress; and higher utilization of health care services such as emergency department and inpatient services (Hser et al. 2006; Mueser et al. 1998; Ziedonis 2004).