Anyone working with transitional youth, individuals aged 16 to 25 years, needs to read this brief. It will help you in understanding their mental health problems, recidivism, and effective transition to adulthood. Sections of this brief include: overview, development during transition to adulthood, and potential pitfalls of the transition age; mental health problems and juvenile justice involvement during the transition age; critical issues facing justice-involved transition age youth with mental health problems—system involvement (child welfare, special education, mental health services, vocational rehabilitation, and independent housing), services for detained and incarcerated youth, and interplay between multiple systems; effective policies and practices for youth with mental health problems; evidence-based and promising practices and policies—Multisystemic Therapy (MST), foster care, wraparound services, diversion programs, reentry and aftercare programs, and coordination of care programs; domain-specific services—mental health treatment, substance use treatment, educational and vocational supports, health care, housing and transportation, and pregnancy and parenting; nine policy and practice recommendations; and conclusion. “Youth with both juvenile justice involvement and mental health problems are a vulnerable group, particularly during the transition from adolescence to adulthood. The multiple problems faced by such youth present barriers to meeting the normative developmental milestones of this age, including vocational and educational success, development of stable relationships, and maturation into productive adults. Current policies and practices in the juvenile justice system are not well suited to meeting the multiple needs of these youth and, at times, can exacerbate existing problems. However, given the high prevalence of youth with mental health problems involved with the juvenile justice system, providers and policymakers have the opportunity to impact a large number of vulnerable youth through the implementation of effective programming in this system. Substantial changes in the juvenile justice and mental health systems will be required to ensure successful transitions to adulthood for this group” (p. 37).