Positive Behavioural Support (PBS) is a multicomponent framework designed to address the multiple factors that influence an individual’s behaviour, with a focus on improving quality of life (Kincaid et al. 2002). PBS, in the UK, has largely been implemented by peripatetic community teams, in developmental disability (DD) services. High prevalence of challenging behaviour in individuals with DD (Jones et al. 2008) is a result of the interaction between vulnerability, environmental and impact factors (Hastings et al. 2013). This thesis will attempt to expand existing knowledge on all three of these aspects in relation to PBS as an intervention model. The thesis begins by presenting a total population study regarding the prevalence of challenging behaviour (chapter 2). This study investigates the associated correlates of challenging behaviour, and is the first study to consider the cumulative association of these correlates and propose ways to measure cumulative risk. Chapter 3 considers an impact of challenging behaviour – the high incidence of psychotropic medication use - and explores prescribing patterns associated with various topographies of challenging behaviour. In chapter 4, population data from the Behaviour Problems Inventory – Short Form (Rojahn et al. 2012) are used to estimate norms, clinically significant change and reliable change statistics. This chapter also provides guidance through examples on how this information can be used in research and clinical practice to enable practitioners to evaluate behaviour change in a more robust manner. Chapter 5 is an evaluation of outcome data from a peripatetic PBS service and, for the first time in research, considers statistically meaningful behaviour change (utilising evidence from Chapter 4), quality of life and social validity impacts. Finally, in chapter 6, findings from the four empirical studies are discussed in relation to theoretical implications. Implications for further research and practice in the field of PBS are proposed in chapter 6.