This thesis aimed to enrich the existing research literature concerning sex offenders with an intellectual disability and to offer direct implications for the treatment of these individuals within the National Offender Management Service1 (NOMS). The thesis concerns the exploration of the beliefs about sex and relationships held by sex offenders who have an intellectual disability. Existing research literature indicates that this is an important and somewhat under-researched area. Indeed, we know that people with intellectual disabilities often experience impoverished or distorted sexual learning experiences and commonly lack knowledge or hold mistaken beliefs about sex and relationships. Additionally, there is evidence to suggest that those who sexually offend often have sex education needs. Yet we know little about the nature of the beliefs about sex and relationships held by sex offenders with an intellectual disability beyond quantitative insights offered by comparative studies. This thesis comprises five empirical studies which aimed to address this research gap. All of the studies employed qualitative methodology and recruited participants from a UK prison. The first study aimed to develop and evaluate a method by which beliefs about sex and relationships held by sex offenders with an intellectual disability could be explored. To do so, an existing sex and relationship beliefs exercise was taken from a NOMS sex offender treatment programme and adapted so that it was suitable for use with individuals who had an intellectual disability