Current law is unclear regarding how to respond to youths' potential deficits in the requisite abilities for being adjudicated when deficits are due to immaturity rather than mental illness or mental retardation. In juvenile courts, where adjudicative competence was not traditionally required for most of this century, a number of States recently have recognized the legal requirement of competence to stand trial in juvenile proceedings, but its meaning is uncertain. The application of legal criteria for competence to stand trial is not the only reason to be concerned about youths' capacities in the trial process. Even youths who meet these legal criteria may present special issues for attorneys who represent them, because their immaturity may influence their participation in their trials in ways that have potential consequences for the quality of their representation. The distinction between formal competence criteria and broader concerns about youths' participation in their trials is outlined and discussed in Part I of this chapter. Part I also identifies the relevant abilities associated with capacities as a defendant, adolescent populations about which the authors are concerned, and areas of theory and research that are relevant for review. Part II then reviews research that describes youths' capacities relevant for their participation in their trials. Part III identifies what can be inferred from the review and suggests implications for policy, law, and practice. 100 references