Bardet-Biedl Syndrome (BBS) is an extremely rare genetic disorder, affecting approximately 1 in 125,000 individuals in the United Kingdom. BBS is characterised by a range of features including degenerative visual impairment, obesity, hormonal problems and developmental differences. It is thought that individuals with BBS are more likely to present with learning difficulties (including meeting the criteria for a learning disability) and characteristics associated with autism. This thesis aimed to further explore the neurodevelopmental profiles of children and young people with BBS through two empirical studies. The aims of these studies were to guide support needs for children with BBS and their families, which have traditionally focused on medical needs. Part A of this portfolio focused on the prevalence and profile of learning disability through direct assessment of cognitive and adaptive functioning in a clinic setting. Part B looked at a broader behavioural phenotype through parent-report questionnaires, with an autistic comparison group. The findings of both studies supported the hypothesis of developmental differences in a BBS population and provided further information about the developmental phenotype, and recommendations for support.