Breathing and respiration are a fundamental and vital processes for our survival. Breathing or pulmonary ventilation can be defined as the biological process of inhaling oxygen (O2) and exhaling carbon dioxide (CO2), while respiration can be defined as the chemical process that occurs within the cells following the gas exchange occurring in the lungs. Given its importance, it seems logical to find the most efficient breathing mode during exercise and should be a focus for many athletes, coaches, and practitioners when seeking to improve aspects of athletic performance. Chapter One of this thesis provides a basic introduction with a definition and description of the physiology (cellular respiration) and mechanics of breathing. The importance of oxygen, specifically during exercise is discussed, followed by some of the limitations of the respiratory system when fatigued. Different breathing methods that have been previously used are introduced such as reduced breathing frequency and nasal-only breathing and their potential for improving aspects of athletic performance. Chapter Two reports on the current literature regarding the use of different breathing modes (nasal, oronasal, and oral) during training. Recently, research has been conducted regarding the use of various breathing modes, alongside respiratory muscle training (RMT) devices and their effects on aspects of athletic performance. Previous studies have observed the acute effects of these breathing modes on athletic performance. Current evidence suggests that nasal-only breathing has the largest acute effect on aspects of athletic performance such as VO2 max, heart rate, and minute ventilation (VE) and shows the greatest potential for performance adaptation. In terms of longitudinal research, there is limited evidence on the effects of a long-term breathing interventions such as nasal breathing. Therefore, the work of this thesis aimed to add to the current literature and further investigate the relationship between breathing and athletic performance, in particular the effects of nasal-only breathing. Chapter Three contains a pilot study that aimed to determine the effects of a 4-week nasal breathing intervention on aspects of athletic performance. As part of the study (n= 10) participants (28 ± 10.12) years were divided into an experimental (n=7) and control (n=3) group. All participants underwent pre and post testing consisting of a maximal ramp test. The 4-week training intervention consisted of three training sessions per week over the 4-weeks, totalling 12 training sessions. Both the experimental and control groups for Weeks-1&2 performed 10-rounds of 10-seconds maximum effort, interspersed with 30-seconds of active recovery. In Weeks-3&4 the number of rounds increased to 15 but the rest intervals remained the same. All sessions were performed on an Airdyne air resisted bike. The experimental group performed all 12-sessions with mouth tape to ensure nasal-only breathing occurred during training, while the control group performed the training with no taping or instructions regarding breathing technique. Analysis revealed no significant effects were observed on maximal ramp test performance, i.e., change in maximum power or VO2 peak in either the control or experimental group. Large but non-significant between-group differences were observed for VE, tidal volume, respiratory compensation point and alveolar ventilation (p-values 0.22 to 0.06). Interestingly, there were notable decreases in breathing frequency and minute ventilation at all submaximal intensities (50-80% VO2 peak; ES: 0.15 to 0.89), with a significant difference in minute ventilation at of 60% VO2 peak, suggesting potential improvements in breathing economy and gas exchange at submaximal intensities. Overall, it was concluded that nasal-only breathing has the potential to improve aspects of athletic performance via training adaptations, specifically during submaximal intensities; however, further research is needed with longer intervention durations and greater participant numbers.