Study findings show that juvenile offenders exhibit significant variation in the severity and persistence of mental health and substance-use problems, with four classes distinguished by the presence and severity of these variables. Class 1 was moderately low in psychiatric symptoms and co-occurring substance-use problems; whereas class 2 was moderately high in psychological distress and related problem behavior. For these two classes, there were significant differences regarding substance-use variables, but these differences were less pronounced compared to the differences in psychiatric symptoms found. Class 3 was relatively free of psychiatric symptoms, but had severe problems with substance use, and class 4 had high levels of psychiatric symptoms, but less severe substance-use problems. The range of the four classes showed pronounced differences between a severely distressed subgroup of 81 youth (11 percent) and a larger subgroup (n=195) of juvenile offenders characterized by mild psychiatric symptoms and mostly experimental substance use and low levels of problem behavior. Of particular concern for the juvenile justice system is the severe class that self-reported high levels of violence, delinquency involvement, substance use, and other problem behaviors. Intensive mental health and substance use services are required for this group. The authors discuss treatment efficiency, policymaking for juvenile offenders, and future taxonomic studies of juvenile offenders. Study participants were drawn from the residential rehabilitation services of the Missouri Division of Youth Services in 2000. This population is representative of incarcerated youth nationally in terms of the average age and gender distribution, percentage of delinquent compared with status offenders, and the rate of State youth currently incarcerated. 4 tables, 1 figure, and 53 references