Historically, almost all defendants found incompetent to stand trial have been persons with mental illnesses or mental retardation. In recent years, an increase in the number of adolescents tried as adults and the number of younger children tried in juvenile courts has raised questions about children’s and adolescents’ capacities to participate in their trials -- not necessarily due to mental illness or mental retardation, but because of intellectual and emotional immaturity. This first-ever large-scale study involved over 1,400 male and female participants between the ages of 11 and 24. The study was conducted in four sites - Philadelphia, Los Angeles, Northern and Eastern Virginia, and Northern Florida -- in order to obtain a sample with cultural, ethnic, and socioeconomic diversity. Half of the participants were in jail or detained in juvenile detention centers at the time of the study, and half were individuals of similar age, gender, ethnicity, and social class but residing in the community. These individuals were administered a standardized battery of tests designed to assess their knowledge and abilities relevant for competence to stand trial, their legal decisionmaking in several hypothetical situations, and measures of a number of other characteristics that could influence these capacities. The results showed that juveniles aged 11 to 13 were more than three times as likely as young adults (individuals aged 18 to 24) to be “seriously impaired” on the evaluation of competence-relevant abilities, and that juveniles aged 14 to 15 were twice as likely as young adults to be “seriously impaired.” Individuals aged 15 and younger also differed from young adults in their legal decisionmaking. Juveniles of below-average intelligence were more likely to be “significantly impaired” in abilities relevant for competence to stand trial than juveniles of average intelligence. These results indicate that, compared to adults, a significantly greater proportion of juveniles in the community that are 15 and younger, and an even larger proportion of juvenile offenders this age, are probably not competent to stand trial in a criminal proceeding.