The ability to perform under pressure is crucial for athletes who wish to optimally execute their skills and training when it matters most. While experience, training, and contextual demands can all determine how an athlete may cope in the face of pressure, one technique that has previously been utilised to assist athletes in such moments is slow breathing. In line with the neurovisceral integration model, it is suggested that slow breathing may aid performance under pressure primarily by reducing psychophysiological stress-responses (e.g., reducing perceived stress and heart rate), thus enabling optimal cognitive functioning during performance such as better decision-making. Testing this assumption, the purpose of this PhD thesis was to examine slow breathing in the context of pressurized performance, and assess its effectiveness on a variety of psychological, physiological, performance-related, and well-being outcomes. More specifically, a systematic review with accompanying effect sizes is presented in Chapter 2. Although limited evidence was found for slow breathing benefitting psychophysiological stress-responses or behavioural performance, results did indicate that slow breathing benefitted cognitive performance, with effects stronger for multi-session than single session interventions. In addition, the review revealed little work on some specific breathing techniques (e.g., diaphragmatic), and that more research was needed to establish whether slow breathing interventions aid behavioural (i.e., sports) performance under pressure. Thus, informed by Chapter 2 findings, three experimental studies are then presented (Chapters 3, 4, and 5), which examined the effects of slow diaphragmatic breathing in three different athlete groups. In Study 1 (Chapter 3, n = 67), a randomised control trial found no conclusive evidence that a single-session of slow diaphragmatic breathing benefitted psychophysiological stress-responses (i.e., stress appraisals, heart rate, vagal tone) or optimised pressurized performance (i.e., pistol shooting accuracy) in novice athletes. Next, in Study 2 (Chapter 4, n = 6) a mixed-methods case study design found promising trends of reduced perceived stress reactivity and enhanced well-being following multiple sessions of slow diaphragmatic breathing in professional elite athletes, although these results were not statistically significant. Additionally, the intervention had mixed effects on physiological markers of stress (i.e., decreased heart rate, decreased vagal tone) and perceived performance. However, via qualitative data collection, participants provided further detail on how the intervention might have benefited well-being and performance, by improved physical states, affective responses, and cognitive functioning, potentially via focused attention. Finally, Study 3 (Chapter 5, n = 4) also utilised a mixed-methods case study design, examining the effects of multiple sessions of slow diaphragmatic breathing in university athletes. Quantitative data displayed promising results, with perceived stress reactivity significantly reduced, and most participants improving performance on a pressurized task post-intervention. However, relatively limited effects were found for stress appraisals and physiological stress-responses (i.e., heart rate and vagal tone). Qualitative findings reported that the intervention might have aided performance and well-being by benefitting attentional, cognitive, and affective processes, and improving sleep quality, potentially through the mechanisms of focused and non-focused attention. In summary, this programme of PhD research extends the literature on slow breathing techniques and their effect on stress-responses, performance, and psychological well-being, particularly by experimentally examining slow diaphragmatic breathing. Overall, results for psychophysiological stress-responses were mixed throughout the research, meanwhile promising, although non-significant effects on performance and psychological well-being were found after multiple sessions of slow breathing. In terms of theoretical contributions, this programme of PhD research failed to provide conclusive support for the neurovisceral integration model (i.e., slow diaphragmatic breathing benefitted emotional and cognitive processes but had limited effect on vagal tone), and refuted claims of the Theory of Challenge and Threat States in Athletes (i.e., that a challenge state is necessary for optimal performance). Future research is encouraged to continue examining the effect of slow diaphragmatic breathing on pressurized performance and including measures of attention, while using robust study designs such as randomized control trials. Finally, in terms of practical application, slow breathing could be a beneficial technique in helping athletes to perform under pressurized conditions by reducing attention to stressors and could buffer athletes from the detrimental effects of long term exposure to stress by providing periods of mental rest and enhancing sleep quality.