This article describes four waves of juvenile justice reform across the past century that have profoundly impacted how youth are served in community-based, detention, and correctional settings. This first wave of reform began in 1899 as Jane Addams founded the modern juvenile court in Chicago. These progressive reforms soon spread worldwide. Children were seen as unique from adults and the mission was no longer to administer punishment but to serve the best interests of the child. The second wave was marked by the 1967 U.S. Supreme Court decision "In re Gault," which gave juveniles due process rights. The court described young offenders as having the worst of both worlds--neither receiving legal protection like adults nor the care and treatment envisioned by the juvenile court. A third wave came in the 1980s as politicians and press stoked public fears of youth violence. What followed was a regression to harsh punitive approaches of the past, and the brunt of these policies fell on children of color. Beginning in 2000, the fourth wave retains some punitive practices but attempts to balance goals of accountability, public safety, and economic efficiency with improved outcomes for children, families, and communities. Instead of more "reform," the authors call for "transformation" built upon solid evidence from the science and practice of positive youth development. They describe how "smart on crime" is replacing the outmoded "tough on crime" mantra. A new science of resilience and positive psychology is propelling the country from deficit-based to strength-based strategies (Bonnie, Johnson, Chemers, & Schuck, 2013; Butts, Mayer, & Ruth, 2005). Transformational leaders are embracing youth-centered, developmentally appropriate approaches (Heller, Ludwig, Miles, & Guryan, 2011; Roush, 2004). The authors assert that it is now time to move these initiatives to scale to meet the needs of the nation's most vulnerable youth.