In response to the lack of universal agreement about what works in probation supervision (Trotter, 2013) we undertook a Rapid Evidence Assessment of the empirical literature. Our analysis of research into the effect of probation supervision reducing reoffending included 13 studies, all of which employed robust research designs, originating in the USA, UK, Canada and Australia, published between 2006 and 2016. We describe the papers included in our review, and the metaanalyses of their findings. Overall we found that the likelihood of reoffending was shown to be lower for offenders who had been exposed to some type of supervision. This finding should be interpreted cautiously however, given the heterogeneity of the studies. We suggest future research and methodological considerations to develop the evidence base concerning the effectiveness of probation supervision.