The focus of this paper is on what works to promote successful reentry for youth who are under juvenile justice supervision, which is a process that begins when a youth first comes into contact with the system. Part One of the paper outlines four core principles that underlie what works in reducing recidivism and improving outcomes for youth in the juvenile justice system. First, base supervision, service, and resource-allocation decisions on the results of validated risk and needs assessments. Second, adopt and implement programs and services that have been demonstrated to reduce recidivism and improve other youth outcomes; and use data to evaluate the results and guide system improvements. Third, use a coordinated approach across service systems in addressing youths' needs. Fourth, tailor system policies, programs, and supervision to reflect the distinctive developmental needs of adolescents. Part Two of the paper identifies the lessons learned from research and practice on implementing these four principles, and examples are provided on how State and local juvenile justice systems have applied the principles in practice. Issues addressed include the determination of how, when, and how often assessments should be conducted; the development of quality standards and support for matching youth to the services available; evaluation of treatment and service outcomes; a structure of ongoing collaboration across government branches and service systems; and a guiding set of values that reflect the latest research on youth developmental characteristics and how best to manage them for youths' benefit. 10 notes