PREVALENCE OF MENTAL HEALTH-SUBSTANCE USE Emerging recognition of mental health-substance use disorders dates back to the 1970s, when professionals became increasingly aware that a signifi cant number of people presented with both disorders.1 Subsequently, numerous studies of both community and patient samples have shown substantial rates of mental healthsubstance use disorders. A large prevalence survey (National Survey on Drug Use and Health; NSDUH) conducted annually in the United States estimates that in 2007 there were 5.4 million adults who had serious psychological distress (SPD)*† and a co-occurring substance use disorder (SUD).5 Th e Epidemiologic Catchment Area study reported lifetime prevalence of 29% for drug dependence among those with a mental disorder and lifetime prevalence of a mental disorder of 53% among those with a drug disorder.6 Similar rates of comorbidity in the US were also found in the National Comorbidity Study.7 A review of several studies8 found somewhat higher prevalences of comorbidity: in mental health settings 20%–50% of clients had a lifetime co-occurring SUD, while among substance abuse treatment clients 50%–75% had a lifetime co-occurring mental disorder. Rates of comorbidity in the UK (where serious mental illness is restricted to psychosis) are generally lower than in the US, 20%–37% in mental health settings, and 6%–15% in addiction settings.9