The present paper sets out to review the literature on several aspects of sex offenders with intellectual and developmental disabilities, including the relationship between sex offending and developmental disabilities, the prevalence and characteristics of sex offenders in this client group, assessment, treatment, and outcome of intervention. Several important variables were identified as influencing the disparate results found in different prevalence studies. These include variations in inclusion criteria, differences in the source of the sample, differences in determination of IQ, the impact of deinstitutionalization, and the effect of changing social and penal policies in the area where studies have been conducted. Although some studies have suggested an increasing incidence, there is no clear evidence for the over- or under-representation of people with developmental disabilities amongst sex offenders. One of the main methodological flaws in several reports listing the characteristics of sex offenders is that considerations are based on clinical samples. Therefore, there is no control group to show that these characteristics do not exist in other samples of individuals with intellectual disability (ID). It does appear that sex offenders with ID are more likely to commit offences across categories and to be less discriminating in their victims. There may also be an association with sexual abuse in childhood. The primary issue considered has been assessment of competency, in that people with ID are considered to be disadvantaged by the criminal justice process. While several authors have delineated the important areas for assessment, there are few assessment measures with robust psychometric properties. Pharmacological, behavioural, educational and cognitive treatments are reviewed. Several comprehensive treatments which include all of the aforementioned methods are also considered. Although most studies do not report particularly positive outcomes, several authors have found better outcomes with treatment lasting at least 2 years.