A key dimension of sexual assault prevention is stopping perpetrators from re-offending (often referred to as tertiary prevention). One way of monitoring effectiveness in this regard has been through the use of recidivism rates, yet results from international studies vary quite dramatically, with recidivism rates of between 3% and 70% being reported. What are the reasons for this variation and what can we reasonably consider recidivism rates to tell us? This summary describes how sexual offender recidivism may be measured and discusses the impact this may have in how rates are used. It includes methodological limitations and the affect of attrition rates in the criminal justice system.