More than three quarters of youth involved in the juvenile justice system have been exposed (usually repeatedly) to traumatic stressors, including abuse or family or community violence, life-threatening accidents or disasters, and interpersonal losses. The prevalence of posttraumatic stress disorder (PTSD) among justice-involved youth is three to ten times greater than in community samples. In addition, justice-involved youth with PTSD are at high risk for problems, including depression and suicidality, oppositional-defiant and conduct disorders, risk taking, and substance abuse. This chapter provides an overview of clinical epidemiology research on PTSD, comorbid emotional and behavioral disorders, and complex traumatic stress disorders associated with the poly-victimization experienced by many youth in the juvenile justice system. Evidence is described of complex forms of PTSD among justice-involved youth that include: (1) persistently reduced adaptive arousal reactions and episodic maladaptive hyperarousal, (2) impaired information processing and impulse control, (3) self-critical and aggression-prone cognitive schemas, and (4) deviant peer relationships that model and reinforce disinhibited reactions, maladaptive ways of thinking, and aggressive, antisocial, and delinquent behaviors. Findings are highlighted concerning PTSD and vulnerable subpopulations, including girls, ethnoracial minority youth, and juveniles charged with sexual offenses. Finally, the chapter concludes with a discussion of trauma-informed approaches for court proceedings, juvenile justice facilities and rehabilitation services, and mental health treatment.