"The best way to help prevent a youth’s subsequent contact with the juvenile justice system is to prevent him or her from being involved with the system in the first place. The field has been engaged in significant efforts to divert status offenders and other low-risk youth from ever coming into contact with the system. The focus of this white paper is on what works to promote successful reentry for those youth who are under the supervision of a juvenile justice system, which encompasses a process that begins the moment any youth comes into contact with the system, no matter how brief or at what level, to support their successful transition from supervision to a crime-free and productive adulthood" (p. 3). This white paper is divided into two parts. Part One-Policies and Practices That Reduce Recidivism and Improve Other Youth Outcomes: Principle 1--base supervision, service, and resource-allocation decisions on the results of validated risk and needs assessments; Principle 2--adopt and effectively implement programs and services demonstrated to reduce recidivism and improve other youth outcomes, and use data to evaluate system performance and direct system improvements; Principle 3--employ a coordinated approach across service systems to address youth’s needs; and Principle 4--tailor system policies, programs, and supervision to reflect the distinct developmental needs of adolescents. Part Two-Key Implementation Strategies, Structures, and Supports: Principle 1--base supervision, service, and resource-allocation decisions on the results of validated risk and needs assessments; Principle 2-- adopt and effectively implement programs and services demonstrated to reduce recidivism and improve other youth outcomes, and use data to evaluate system performance and direct system improvements; Principle 3--employ a coordinated approach across service systems to address youth’s needs; and Principle 4--tailor system policies, programs, and supervision to reflect the distinct developmental needs of adolescents.