Restricted and repetitive behaviours and interests (RRBIs) are a broad and heterogeneous class of behaviours that are present in a number of neuro-developmental conditions, such as, autism spectrum disorder (ASD) and Prader-Willi syndrome (PWS), but also feature in early typical development. The management of maladaptive RRBIs may present a considerable challenge to parents and the behaviours themselves may hamper the acquisition of new skills in affected individuals. Despite early research being fraught with lack of consensus in terminology and item overlap on standardised measures, the past decade has seen a resurgence of research in the area. Most systematic research, however, has been conducted in children with ASD and there are few studies that attempt to make links across disorders. Furthermore, debate over the structure of RRBIs and the use of domain scores in research has led to inconsistencies in studies attempting to identify their neuro-cognitive correlates. The collection of studies in this thesis attempt to move away from domain scores of repetitive behaviour by using factor analysis to parse the RRBI domain into homogeneous behavioural factors. These factors form the basis of studies in later chapters examining the relationship between RRBIs and executive functions (EF) in ASD, PWS and typical development. Results show that there is a differential pattern of association between different types of EF and specific RRBIs and findings across groups indicate possible common executive correlates of behaviour.