One of the pillars of the American justice system is the assurance that those who stand accused of crimes be mentally competent to understand and participate in their trials. The conventional standard for competence has typically focused on the effects of mental illness or mental retardation on individuals’ capacities to grasp the nature of their trials or their abilities to decide how to plead. Yet as the courts, both juvenile and adult, see increasingly younger defendants some argue that the law should also take into account adolescents’ lesser capacities owing to emotional and psychological immaturity. This brief details findings from the first comprehensive assessment of juvenile capacities to participate in criminal proceedings using measures of both trial-related abilities and developmental maturity.