Research indicates that many youth who come into contact with the juvenile justice system may have mental health and substance use related disorders. Problems related to these conditions play a continuing role in delinquency and pose risks to the welfare of youth, juvenile justice staff, and others. Identifying troubled youth is the first step in providing them with appropriate treatment. Early, accurate identification of youth with mental disorders in the juvenile justice system is a critical need. Once identified, these youth can receive the services required to improve their lives, reduce recidivism rates, and promote community safety. To take that first step, juvenile justice professionals need reliable screening and assessment instruments and practical guidance in their effective use. This Resource Guide is intended as a basic tool for juvenile justice professionals working toward this goal and offers a comprehensive, user friendly synthesis of current information on instruments that can be used to screen and assess youth for mental health and substance use related disorders at various stages of the juvenile justice process. The Guide includes profiles of more than 50 instruments, guidelines for selecting instruments, and best practice recommendations for diverse settings and situations. It also offers clinicians and other professionals working with youth in the juvenile justice system a range of best practice information that will assist in better identifying youth with mental health disorders, thus ultimately improving their treatment. Reviews and summaries of information about the most effective instruments for screening and assessing youth for mental health and substance use disorders at various points in the juvenile justice system are included and it also provides examples of a variety of models and approaches that have been developed to use available instruments.