Offenders in prisons in England and Wales have access to a suite of accredited interventions designed to reduce reoffending. The programmes offered are generally cognitive-behavioural in nature, and address a range of offending behaviours and maladaptive thinking styles. This chapter focuses on how programmes are evaluated in a prison setting across England and Wales. The meta-analytic method has been a useful tool in the What Works area. Once a research question has been set there are many issues to address to optimize the success of an evaluation. These include the choice of methodology, of outcome, of outcome measurement, of comparison group and analysis. The chapter describes the evaluation approach followed in the UK where the National Offender Management Service (NOMS) Interventions Group (IG) routinely collects both demographic and criminal variables on every offender undergoing treatment. It also examines the outcome studies pertaining specifically to Her Majesty's Prison Service (HMPS) programmes.