The millennium marked the beginning of a second century for the formal system of juvenile justice in the United States. From its inception, the central focus of the system has been delinquency, an amorphous construct that includes not only "criminal" behavior but also an array of youthful actions that offend prevailing social mores. Thus, the meaning of delinquency is markedly time dependent. Likewise, methods for addressing the phenomenon have reflected the vagaries of social constructions of youth and youth deviance. American juvenile justice was founded on internally conflicting value systems: the diminished responsibility and heightened malleability of youths versus individual culpability and social control of protocriminality. During its first century, the latter generally have become increasingly predominant over the former. Those most caught up in the system, however, have remained overwhelmingly our most marginalized youths, from immigrants' offspring in the early 20th century to children of color in contemporary society. The implications of such theoretical and sociodemographic variations are considered, and their implications are reviewed for public policy beyond mere political symbolism.