How to deliver treatment to people who have both a substance use disorder and mental disorder is a problem for all health services. Research on service delivery is scarce, turf wars are common, and people with comorbid mental disorders and substance use disorders often fall through the cracks in the separate service systems. Discussion papers from countries with health service structures as diverse as the United Kingdom, the United States of America, Australia, and The Netherlands all indicate that persons with co-occurring drug and alcohol and mental disorders are failing to access treatment or are being poorly treated by the current systems.

How does the service system respond? There is ill-defined literature regarding the implications of comorbidity on treatment and service provision. Very few studies have systematically and empirically reviewed treatment or service delivery options, although there is an increasing number of discussion papers (e.g., Bellack & Gearon, 1998; Gournay, Sandford, Johnson, & Thornicroft, 1997; Jerrell, Wilson, & Hiller, 2000; Kavanagh, 1995; Kessler, 1995; Mueser, Bellack, & Blanchard, 1992; Mueser, Drake, & Miles, 1997; Ries, 1992; Smith & Hucker, 1994).

These discussions of service delivery often focus on people with psychoses and substance use, usually cannabis use; those whose disorders cause obvious need for treatment, and those who the community cannot help but notice. Individuals with the more common depressive, anxiety and alcohol use disorders are often overlooked. Yet these disorders comprise some of the most prevalent mental disorders in our community and they cause considerable disability (see Chapter 3 by Andrews et al.; and Burns & Teesson, 2002).

This chapter reviews the evidence for effective service delivery to people with comorbid disorders. It argues that if the burden of comorbidity is to be addressed, it is essential that research on service delivery is conducted and that services are delivered to those with the more common disorders, as well as those with the more affronting yet less common disorders. These disorders have different prevalence rates, treatment responses and require different service delivery responses. We first outline the impact of comorbidity on the course of illness and use of services.