A systematic review of the reasons why detained adult offenders fail to attend or successfully complete treatment programme(s) was conducted. An initial search of the literature identified 2,827 articles, which following evaluation against explicit inclusion/exclusion criteria and a quality assessment was reduced to 13 studies. Extracted data from the 13 studies were synthesised using a qualitative approach. Despite the 13 studies being heterogeneous in design, there was consensus on the reasons offenders gave for completion/noncompletion of treatment. The majority were consistent with the factors outlined in the Multifactor Offender Readiness Model (MORM) and included a perceived lack of self-efficacy, negative perceptions of treatment, staff and peers, an inability to regulate emotions, and a lack of perceived choice and control. A lack of opportunity to engage in established, professionally run, groups, as well as perceived inadequate support from members of staff, was also associated with poor engagement and noncompletion of treatment.