We conducted a systematic review of studies reporting on the effectiveness of Circles of Support and Accountability (Circles). Circles use volunteers to provide support for sex offenders living in the community. We searched 10 databases up to the end of 2013 and identified 3 relevant outcome studies. An additional 12 papers or reports were identified by searching reference lists, Google, and contacting key authors and Circles providers to obtain unpublished data. These 15 studies comprised one randomized controlled trial, three retrospective cohorts with matched controls, and 11 case series. The majority reported measures of recidivism, particularly reconviction. The 4 studies with controls generally reported that participation in Circles was associated with lower recidivism although there were few statistically significant differences. Few studies examined changes in risk or psychosocial outcomes. A number of methodological issues are discussed. Longer term, prospective follow-up studies with control groups are required to address these issues.