On any given day, over 100,000 youth are held in custody in juvenile justice facilities across the country, either awaiting trial in detention centers or having been placed in residential facilities after being convicted of delinquencies. A growing body of research suggests that most of these youth meet criteria for at least one mental disorder, and that at least one out of every five have what is considered to be a serious mental disorder often coupled with a co-occurring substance use disorder. Essential to responding to a youth's mental, emotional, and substance use problem is the identification of their problem. Detecting potential mental health and substance use disorders among youth requires reliable and valid screening and assessment instruments, and information on how best to implement the available instruments. This Research and Program Brief is designed to provide clinicians and other professionals working with youth in the juvenile justice system with information about the most effective instruments to use to screen and assess for mental health and substance use disorders among youth at various points in the juvenile justice system. (Contains 10 references.) (GCP)