Background: The prevalence of disability increases with age; therefore with an aging population, interventions to reduce disability are crucial. This thesis adopts a behavioural conceptualisation of disability. The theoretical frameworks of the International Classification of Functioning, Disability and Health (ICF), the Theory of Planned Behaviour (TPB) and the integrated ICF/TPB model are applied to investigate disability and physical activity (PA) behaviours. The thesis aims to: (1) identify the factors involved in the prioritisation of patients for total joint replacement; (2) classify patient pre-operative expectations of total hip replacement (THR) and investigate the relationship between expectations and recovery after surgery, and; (3) test whether the TPB and theory-based interventions can predict and explain PA within individuals. Method: Five studies were conducted. In the first study, health professionals judged whether the items from two prioritisation tools measured each of the ICF constructs. In the second study, surgeons ranked patient vignettes, which differed by constructs from the integrated model, in order of priority for THR. In the third study, a large cohort of THR patients reported expectations of surgery pre-operatively. Health and functioning were also reported pre-operatively and 1-year post-operatively. The fourth and fifth studies were a series of experimental n-of-1 studies using diary methods assessing TPB cognitions and PA behaviours. Results: There is a lack of agreement between judges in relation to the content of many of the items from prioritisation tools. Behavioural and psychological factors can influence prioritisation for THR. The majority of patient expectations of THR addressed activities and social participation; however, the evidence for a relationship between expectations and recovery was limited. The TPB can predict PA within some individuals but the evidence in support of interventions to increase PA was limited. Discussion: The findings provide important clinical and theoretical implications for understanding disability and physical activity behaviours.