In recent years, data from the United Kingdom (UK) has suggested that around 3% of all reported offences are of a sexual nature. A proportion of these offences are committed by young people (under 18-year olds) with intellectual and developmental disabilities (IDD). Yet, the collective empirical understanding and knowledge of the mechanisms that lead young people with IDD to display and commit such behaviours, is sparse. In comparison to their non-disabled peers, very little is known about the characteristics of the young people with IDD who come into contact with specialist harmful sexual behaviours (HSB) services. In part this is due to a lack of appropriate assessment instruments available to researchers and clinical practitioners within UK. The aim of the present thesis was to explore current assessment practices for young people with IDD across forensic service providers, to identify limitations and implications of such shortcomings in our clinical understanding, and to adapt and validate a selection of instruments. The majority of the offences are committed by adult males, with a small proportion accounted by men with IDD. Furthermore, around a third of sexual abuse reported by children and adolescents, in the UK, involves another young person under 18 years of age as the perpetrator. However, a general expert consensus, agrees that these figures are most likely under-estimates of the true scale of the problem. Furthermore, up to 40% of young people within specialist service providers for HSB have been suggested to have IDD. Again, this represents another contested figure, considered too high. It is suggested to stem from poorly designed studies, indicating very little is known about these vulnerable young people. While their presence has clinical implications within HSB service providers, empirical data is scarce, and the area has received little attention in comparison to non- disabled counterparts. Review of prior literature suggests adults with IDD who commit sexual offences are a distinct subgroup, different to the general sexual offender cohort. This is even more true for adolescents displaying HSB. In general, young people are different to adults as they are not only going through puberty, a time of rapid development of both physical and psychological changes, but they, unlike adults, lack privacy and still rely on a wide network of agents such as parents, educational providers and professionals which they have little impact and control over. Research also indicates, for a significantly larger cohort of those who present with HSB there is a higher prevalence of adverse childhood experiences, than for the age matched peers. Studies in variations of types of abuse and neglect that might be experienced by young people who display HSB versus more general delinquents is ongoing, but a general consensus states that maltreatment events in childhood affect the young person's style of coping with psychosocial stressors and has a potential significant impact on their psychosocial development. Some of the more recent studies have suggested that young people with IDD, who display HSB, are more likely to have experienced abuse and neglect, with the impact and the role these experiences play for the individuals often overlooked in the assessment and intervention work provided for the HSB. As demonstrated via the systematic review, in Chapter 3, part of the issue is a national scarcity of appropriate assessment instruments, that have been either developed for or adapted for young people with IDD. Instead, as explored in Chapter 4, service providers are either relying on the use of existing tools, that are inappropriate for young people with IDD, and/or they have developed an over- reliance on risk assessment tools. Subsequently, in Chapter 5 justifications were provided for the adaptation and use of three instruments specifically focused on exploring young people's socioemotional development. The tools selected assess: i) attachment security of the young person to a caretaker, ii) their sense and experience of loneliness, and iii) provide an insight into their resilience. The three tools were chosen, in part due to them being in current use across service providers (as found in Chapter 4), as well as a need to balance out the risk focused assessment practices, that can overlook the socioemotional needs of the young people. Finally, Chapter 6 explored the psychometric properties of the two adapted instruments and checked the validity of a third tool. The studies in this thesis suggests, more empirical and clinical attention needs to be provided to the area of instrument development and adaptations for young people with IDD. They are a distinct cohort, whose vulnerabilities and disabilities might play a unique role in understanding their offence pathways leading to displays of HSB. Implications for future research and practice are discussed.