The author reviews the significance of the presence of mental illnesses to the juvenile justice system. He acknowledges the absence of a satisfactory system for classifying mental disorders among children and adolescents and sketches what so far is known about certain childhood disorders which appear to have a special significance for delinquent behaviour. He reviews studies on the overlap between delinquency and mental illness amongst young people and argues for the need for standardized screening tools for assessment of young people. In particular, he explains the Massachusetts Youth Screening Instrument (“the MAYSI”). He argues that significant opportunities for learning more about, and treating, mental illnesses among juvenile offenders accompany the increasingly punitive trends in United States criminal justice.