Criminologists have traditionally considered adult-onset offending to be a rare phenomenon (Eggleston & Laub 2002). Consequently, little criminological theory, research or policy has focused on adult-onset offending. However, an emerging body of research suggests that a substantial number of offenders have their first contact with the criminal justice system (CJS) at 18 years of age or older (Delisi & Piquero 2011). Despite increasing interest in adult-onset offenders, the nature of adult-onset offending is still poorly understood. Moreover, it is unclear whether traditional criminal justice responses for adult offenders are appropriate for adult-onset offenders. In this study, the extent, nature and costs of adult-onset offending are examined, alongside the appropriateness and cost-effectiveness of current criminal justice responses.