In many communities throughout North America, researchers have estimated that the majority of juvenile offenders have one or more diagnosed or undiagnosed mental disorders. Recent studies have indicated that at least two-thirds of juvenile detainees have one or more mental disorders, and as many as 20 percent have a severe disorder. Various types of mental disorders do increase the risk for problem behaviors that can bring a juvenile in contact with the juvenile justice system. Youth who suffer from depression, for example, often "act out" their depression through disruptive and aggressive behaviors. Also, juveniles with attention-deficit/hyperactivity disorder often have poor school performance, poor social relationships, poor problem solving skills, and difficulty in attending to stimuli, all of which may be linked to delinquent behavior. Learning disabilities and posttraumatic stress disorder may also be linked to offending behaviors. A section of the chapter discusses the assessment of juveniles for mental disorders. The assessment instruments profiled are the Massachusetts Youth Screening Instrument and the Brief Symptom Inventory. Descriptions of effective treatment approaches encompass cognitive-behavioral approaches, education, motivational interviewing and solution-focused treatment, functional family training, multisystemic therapy, community-based alternatives, and "wraparound" programs. The latter programs provide an array of formal and informal services to juveniles and their families while maintaining the youths in their community. Contraindicated treatment approaches that may even be harmful for mentally ill youth are residence in adult facilities, boot camps, incarceration alone, and unspecified and nondirective counseling methods. 49 references