Background: Chronic diseases are a global issue with high human and economic costs and consequences. Chronic disease treatment requires people to change their health related behaviours with support from health care professionals. Behaviour change science is a component of health psychology and behavioural science that can help explain and predict behaviours to guide and inform intervention design. Dietitians in primary care are well-positioned to support adults with chronic disease to change their diet through nutrition education and counselling that incorporates behaviour change science. The way in which this knowledge is applied within dietary interventions, how it can be addressed in practice and whether behaviour change science is adequately taught within dietetics education programs in Australia is unclear, warranting further investigation. The overall aim of this doctoral research program was to explore the application of behaviour change science in the dietetics profession. This exploration was achieved through the following three aims: i) to investigate how behaviour change science is used in dietetics interventions and determine its effectiveness; ii) to explore how behaviour change science can be applied on an individual level for chronic disease management; iii) to investigate and explore how behaviour change science has been taught within dietetics education programs. These three aims were addressed throughout four phases of research which aligned with the Joanna Briggs Institute (JBI) Model of Evidence Based Healthcare. The JBI Model contextualises five ‘inner segments’ that conceptualises evidence based health care and includes Global Health, Evidence Generation, Evidence Synthesis, Evidence Transfer, and Evidence Implementation. The methodological approach for the body of research drew on both qualitative and quantitative approaches, guided by a pragmatic paradigm with relational epistemology using sequential explanatory mixed methods. This thesis is presented in eight chapters which include a mixture of research chapters and publication papers. This thesis introduces the background and significance of the problem (Chapter 1), reviews the relevant literature (Chapter 2), describes each phase of research with their submitted or published manuscripts (Chapter 4-7), and summarises the body of research and provides recommendations for future work (Chapter 8). Phase One, Study 1, a systematic review, was conducted to explore theory-based dietary interventions delivered by dietitians in primary care settings. This systematic review aligned with the Evidence Synthesis segment of the JBI Model. Thirty papers representing 19 randomised controlled trials and 5172 adults were synthesised. Thirteen studies showed significant intervention effects for the study’s primary outcome, which related to a range of health conditions. Of those studies, 11 were underpinned by the social cognitive theory, a well-known behaviour change theory commonly used in group education settings. There was fair grade evidence to support behaviour change theory, particularly social cognitive theory, to underpin the design of interventions delivered in the primary care setting. The behaviour change techniques explicitly reported by the studies were analysed, with a range of types and numbers used. These findings demonstrated that the reporting of behaviour change techniques could be enhanced for intervention replicability. Furthermore, only a few theories were used to underpin dietitian-delivered dietary interventions within primary care settings. The findings presented an opportunity to explore how other behaviour change theories could be used to understand dietary change on an individual level in the primary care setting outside of clinical intervention settings. Phase Two, Study 2, a qualitative study of adults with Type 2 Diabetes (T2D), was conducted to explore the psychological processes of dietary behaviour change after a chronic disease diagnosis. This study aligned with the Evidence Generation segment of the JBI Model. Twenty-one semi-structured interviews were conducted with adults in Australia who had consulted with a dietitian following a diagnosis of T2D. The integrated behaviour change model guided a deep exploration of the psychological processes involved in dietary behaviour change. The participants expressed a range of motivational, volitional and implicit processes that influenced their decisions to change their dietary behaviours. Motivations stemmed from the desire to improve their health, using support systems to help with life changes resulting from their diagnosis, feeling an ability and responsibility to make changes, and use their diagnosis as motivation to change. Participants expressed the importance of recognising habitual eating behaviours and unlearning them. These findings illustrate how the integrated behaviour change model can deepen our understanding of dietary behaviour change, particularly within the crucial period after a diabetes diagnosis, and can form the basis of future interventions targeting dietary behaviour change.Phase Two, Study 3, a qualitative study, further analysed the 21 transcripts using the Behaviour Change Technique Taxonomy (v1) to identify the self-reported techniques enacted by participants and the technique they perceived their dietitians to use in consultations. This study aligned with the Evidence Generation segment of the JBI Model. The participants described techniques relating to behavioural practice, suggesting almost all the participants had made changes to their dietary intake following a T2D diagnosis. As seen in Study 2, these participants expressed a range of motivations that influenced their decisions to change their dietary behaviours and volitional processes reflected in the behaviour change techniques. The participants reported that their dietitian implemented behaviour change techniques similar to nutrition education and advice. However, there appeared to be less focus on behaviour change techniques relating to self-identity and environmental restructuring. Findings from this study highlighted the multifaceted nature of dietary behaviour change and how behaviour change techniques can be used to gather how patients perceive the nutrition care delivered by dietitians. Future research is warranted to measure the effectiveness of the behaviour change techniques. Phase Three, Study 4, a document review of 18 dietetics education program curricula and 15 interviews with dietetics academics, explored how behaviour change science is taught within tertiary dietetics education programs across Australia and New Zealand. This study aligned with the Evidence Generation segment of the JBI Model. The findings illustrated the fundamental role of behaviour change science in dietetics teaching. However, the dietetics academics reported that the content-laden curricula and need to meet accreditation requirements prevented including more behaviour change science teaching and learning. Assessment types and topics taught varied between programs. These findings provided important foundational knowledge regarding behaviour change science within dietetics education programs. Key areas of improvements in university teaching and learning were identified, including having more practical simulations, scaffolding the behaviour change science content from early years, and integrating the content across courses. Conclusion: The four phases of this doctoral research program presented an in-depth understanding of how behaviour change science applies to dietetics. Dietary interventions delivered in the primary care setting are recommended to be underpinned by behaviour change theories and models to enhance patient outcomes. However, the reporting of important intervention components such as behaviour change techniques need to be considered and made more explicit for replicability. Dietitians consulting in primary care settings can use behaviour change science in their practice to understand patient dietary behaviours and develop patient-centred intervention strategies. The Behaviour Change Technique Taxonomy (v1) is a useful tool that dietitians can use in practice as evidence based strategies. At a university level, there are areas to improve how students learn about behaviour change science, such as consistency in content taught and more detailed assessments. Future research needs to explore dietitians’ perspectives of behaviour change science application to their practice to provide optimal nutrition care to patients with diet-related diseases.