This chapter deals with youth crime and youth justice: offending behaviour committed by children and young people and their subsequent treatment in the justice system. It considers the argument for a bespoke understanding and response to youth and crime as distinct from offending behaviour committed by adults. The discussion begins by looking at how the concepts of ‘childhood’ and ‘youth’ have been theorised and socially constructed over time. The chapter then examines how youth crime and ‘delinquency’ have been explained in individualised, developmental, and agentic terms; how young people may grow into crime, with particular emphasis on the role of culture in deviance; and the link between radicalisation and youth crime. It also describes the dominant formal responses to youth crime before concluding with an overview of progressive, contemporary approaches to delivering youth justice/responding to youth crime, namely, diversion and positive youth justice.