D. Wayne Osgood, E. Michael Foster, and Mark E. Courtney examine the transition to adulthood for youth involved in social service and justice systems during childhood and adolescence. They survey the challenges faced by youth in the mental health system, the foster care system, the juvenile justice system, the criminal justice system, and special education, and by youth with physical disabilities and chronic illness, as well as runaway and homeless youth. One problem is that the services these vulnerable populations receive from these systems as children and adolescents often end abruptly as they transition to adulthood, even though the need for them continues. Youth must leave systems tailored for clients their age and, if they are eligible for further services at all, enter adult systems that are not equipped to address their needs. One exception is the special education system, whose services extend into early adulthood and are designed for individuals' needs. The authors review current public policies directed toward vulnerable youth in transition and find problems in four areas: eligibility criteria that exclude youth from services that might benefit them, inadequate funding for transition services, a lack of coordination across service systems, and inadequate training about young-adult developmental issues for service professionals. The authors then discuss policy options that can help create a developmentally appropriate and socially inclusive system of support for vulnerable youth. Among the options are strengthening all programs for youth in transition, improving the existing systems of care for children and adolescents, addressing the loss of access to services at the age of majority, and coordinating today's multiple systems into a single coherent system. The authors see heightened governmental interest in better supports for vulnerable young adults, both through expanding the federal role in their lives and through improving coordination of the systems that serve them. The Fostering Connections Act of 2008, for example, extended services to adolescents in foster care from the age of eighteen to the age of twenty-one. (Contains 60 endnotes.)