There has been considerable recent debate about overcrowding in UK prisons. The system is struggling to accommodate the growing number of young offenders being given custodial sentences. This POSTnote sets out the scale of the problem and looks at recent trends in sentencing, with a particular focus on young offenders. It examines the factors linked with offending and asks whether better understanding of these can be used to target early actions aimed at deterring young people from offending. Finally, the note examines alternatives to custodial sentencing and assesses how successful these have been in practice.