Social networks matter to addiction researchers and treatment providers. They are an integral part in understanding how and why people do the things they do. For persons with co-occurring substance use problems and mental illness, the need to understand their social networks is very timely, as persons with Co-occurring substance use Disorders and mental illness (CODs) represent one of the hardest groups to reach among those in need of treatment services and cost more than others to treat. Examining the social networks of persons with CODs may assist in understanding why some persons seek treatment services, while a great many others do not. This paper outlines how social networks can offer insight into the dynamic processes associated with entering and persisting through treatment among persons with CODs. The importance of co-occurring disorders Persons with Cooccurring substance use Disorders and mental illness (CODs) represent a very sizable portion of individuals in need of mental health treatment. The National Epidemiologic Survey on Alcohol-Related Conditions (NESARC) shows that 18.9% of individuals with a 12-month alcohol use disorder also report a co-occurring mood disorder, with 17.1% reporting a co-occurring anxiety disorder