One of the more robust findings of the National Comorbidity Survey (NCS) and its replication (NCS-R) is that a relatively large number of co-occurring psychiatric and substance use disorders are concentrated in a relatively small but significant number of individuals. Persons with co-occurring substance use and psychiatric disorders (CODs) are also prone to having a variety of serious and related medical conditions. In some instances, co-occurring medical conditions such as hepatitis C, endocarditis, and HIV infection are direct consequences of behaviors associated with chronic substance use such as injection use. In other instances, the causal pathway may be more indirect and related to limited health care access, repeated exposure to infectious diseases such as tuberculosis, engaging in high-risk sexual behaviors, medication nonadherence, or living in impoverished and blighted environments where violence and its traumatic sequelae are relatively common. Although many studies have documented the increased incidence and prevalence of individual classes of disorders in criminal justice populations, less research has been done to examine the co-occurrence of medical, psychiatric, and substance use disorders among offenders. In order to provide coordinated care that spans institutional and community boundaries as well as treatment and fiscal boundaries, it is necessary to have a clear understanding of the prevalence of co-occurring health conditions, how they interact to sustain illegal and unhealthy behaviors, current treatment utilization patterns, and barriers to treatment access. This study sought to examine these complex, interrelated issues through conducting extensive, structured interviews with a sample of adult detainees in residential psychiatric treatment at the Cook County Department of Corrections (CCDOC). This population represents those with the most serious psychiatric problems managed by the jail system in Cook County and likely those with very high rates of substance use who disorders and, we believe, associated medical conditions.