Studies have shown that approximately 25 percent of youth entering the juvenile justice system have mental health needs. The prevalence of symptoms of mental disorders is greater among youth in higher security levels, and it is higher for girls than boys. Also, between 70 percent and 90 percent of youth in the justice system report lifetime use of alcohol and/or illicit drugs; approximately 50 percent have indicated substance use in the past year. Mental disorders and substance abuse often co-occur in youth involved with the juvenile justice system. Most States have developed or are developing mental health assessments for youth entering the juvenile justice system. To date, these efforts have been limited mostly to screening processes that identify problem symptoms. The chapter lists the questions that should be asked when choosing an assessment instrument. Substance abuse screening instruments enable juvenile justice personnel to assess the likelihood of a substance abuse problem. Such instruments are short, easily administered, and effective in identifying those who may benefit from a comprehensive diagnostic evaluation. Although effective standardized instruments can aid in identifying youths' mental health and substance abuse needs, they cannot substitute for trained staff who understand critical clinical issues. The juvenile justice system has not traditionally developed roles for such clinicians. Further, treatment program development is also needed. Assessment instruments and protocols are only useful if those identified with problems can be referred to treatment programs matched to their needs. 1 figure, 88 references, and a list of 9 resources