A comprehensive understanding of the prevalence of psychiatric disorders among juvenile detainees is an important step toward meeting their needs. Although epidemiological data are key to understanding the psychiatric disorders of juvenile detainees, few empirical studies exist. This chapter lists studies published in the United States since 1990 that examined the diagnostic characteristics of incarcerated and detained juveniles. The Northwestern Juvenile Project was designed to overcome the methodological limitations in two ways. Four directions for future research are recommended: pathways to co-morbidity, studies of females in the juvenile justice system, longitudinal studies, and studies of vulnerability to posttraumatic stress disorder (PTSD) in high-risk youth. Research findings indicate that a substantial number of youth in detention need mental health services. However, providing services within the juvenile justice system poses a number of challenges: screening for mental health needs, providing services, community linkages, and avoid retraumatizing youth.