Obstructive Sleep Apnoea (OSA) is a common sleep disorder for which Continuous Positive Airway Pressure Therapy (CPAP) is the standard treatment. Despite the effectiveness of this treatment, CPAP acceptance and adherence rates are generally suboptimal. The aim of this dissertation was to validate a new, inclusive Health Belief Model (HBM) for the prediction of CPAP acceptance and adherence. It was argued that the HBM could predict future adherence even before patients have experienced the treatment, thus providing valid intervention targets to improve CPAP acceptance as well as adherence. Modifiable constructs that are proximal to the decision making process for OSA patients, were identified from the literature (Chapter 1). Along with the generic HBM constructs of perceived benefits, barriers, severity, risk, self-efficacy and cues to action, additional predictors were incorporated in the OSA specific model. These included biomedical indices of objective disease severity and measures of psychological distress. Study One assessed 77 newly diagnosed, CPAP naïve OSA patients on a questionnaire battery at baseline (prior to CPAP treatment). The questionnaire included HBM measures which were available in the literature; benefits perception, self-efficacy, functional severity, and perceived risk of negative health outcomes. CPAP adherence was assessed at four month follow-up. This initial investigation found that health beliefs alone explained 21.8% of the variance in CPAP adherence, whilst health beliefs and biomedical indices together explained 31.8% of the variance in CPAP adherence. The greatest proportion of CPAP adherence was explained by higher benefits perception, greater severity and lower risk perception. Study Two reported on the development and validation of a measure of the barriers construct (as no measure of this existed). A sample of 113 newly diagnosed, CPAP naïve OSA patients completed a questionnaire containing potential items of the Barriers to CPAP Use questionnaire (BACQ) at baseline. Exploratory Factor Analysis (EFA) showed a two factor structure of the eight-item BACQ, with ‘Barriers’ and ‘Cost of Treatment’ subscales identified. The BACQ had an internal consistency of 0.82 and readability at a fifth grade reading level. The aim of Study Three was to develop a valid and reliable measure of the cues to action construct. A sample of 63 OSA patients (from the total 113 patients assessed in Study Two) completed a questionnaire containing potential items of the Cues to CPAP Use questionnaire (CCUQ) one month after being prescribed CPAP. EFA showed a three factor structure of the nine-item CCUQ, with ‘Health Cues’, ‘Partner Cues’ and ‘Health Professional Cues’ subscales identified. The CCUQ demonstrated modest internal consistency and split-half reliability, and readability at a seventh grade level. Study Four incorporated three sub-studies which assessed the accuracy of the fully articulated HBM (including the BACQ and CCUQ measures) in predicting CPAP adherence at two months. Study Four A reported on the same sample of 113 CPAP naïve patients (from Study Two). Structural Equation Modelling demonstrated the complex relationship between health beliefs, psychological variables, and biomedical indices in CPAP adherence. The full HBM predicted 24% of the variance in CPAP adherence at two months. Adherence was directly predicted by lower perception of treatment cost as a barrier to CPAP use, higher self-efficacy, and higher BMI. Study Four B reported on 63 patients who completed HBM questionnaires at one month. The model predicted 42% of the variance in CPAP adherence at two months. Adherence was directly predicted by greater benefits perception, greater psychological distress, and lower perception of the Health Professional as the important cue to action. Study Four C investigated changes in health beliefs between baseline and one month. By one month patients generally reported more positive attitudes to CPAP, and better overall functioning. These changes did not correlate with CPAP adherence. Those who used CPAP more than four hours per night demonstrated greater improvement in functional severity and in anxiety score. Study Five used the findings of the HBM studies in developing a theory-driven Motivational Intervention (MI) to target specific beliefs associated with poor adherence. 101 newly diagnosed, CPAP naïve OSA patients were randomly assigned to nurse-led MI + Standard Care (50 participants), or to Standard Care only (51 participants). MI patients received two sessions before starting CPAP, and one session one month after CPAP prescription. By three months, the MI group used CPAP 50% more of the time, and were six times less likely to reject CPAP. The MI group demonstrated greater self-efficacy and a lower perception of barriers to CPAP use. The findings were largely supportive of the HBM theory of CPAP adherence. The substantive findings of this dissertation were that patients do need relevant, timely and targeted support in order for them to effectively commence on CPAP and then continue to adequately adhere. At a minimum, psychological intervention, such as a Motivational Intervention, is likely to be needed for the subset of patients who report beliefs associated with poor adherence at pre-treatment.