Abilities associated with adjudicative competence were assessed among 927 adolescents in juvenile detention facilities and community settings. Adolescents' abilities were compared to those of 466 young adults in jails and in the community. Participants at 4 locations across the United States completed a standardized measure of abilities relevant for competence to stand trial (the MacArthur Competence Assessment Tool--Criminal Adjudication) as well as a new procedure for assessing psychosocial influences on legal decisions often required of defendants (MacArthur Judgment Evaluation). Youths aged 15 and younger performed more poorly than young adults, with a greater proportion manifesting a level of impairment consistent with that of persons found incompetent to stand trial. Adolescents also tended more often than young adults to make choices (e.g., about plea agreements) that reflected compliance with authority, as well as influences of psychosocial immaturity. Implications of these results for policy and practice are discussed, with an emphasis on the development of legal standards that recognize immaturity as a potential predicate of incompetence to stand trial.