Chapter One reviews the differences between performing in temperate and hot and humid environments. When compared to performance in temperate environments, heat-stressful environments acutely alter physiology heart rate, sweat rate, both core and skin temperature, plasma volume and local blood flow. Heat acclimation (HA) protocol introduced prior to competition in varying ways specific to the competition type has been shown to have large impacts on eventual performance, often summarised through heat response tests (HRT) or adaptations of thermoregulatory physiology. Individual responses to HA have been proven to vary between individuals with little evidence presented to explain why such variation occurs. In Chapter Two, 10 endurance-trained competitive triathletes (aged 33.6 ± 10.6 years, 8 males and 2 females; VO2 max 56.9 ± 11.1) completed a 14-day, cycling-based HA protocol (36 °C, 65% relative humidity). Participants completed two HRTs (Day 1 & Day 13), composed of 20 minutes at a steady state intensity immediately followed by a 30-kilometer time trial. Between these HRTs were seven HA sessions. Performance in the 30-kilometer time trial significantly increased as a result of the HA protocol (p = 0.037). Two regression models for predicting performance outcomes were generated using stepwise regression analysis. A ‘best overall fit’ model using baseline VO2, sweat loss, and sweat sodium composition explains 53% of the variability of performance improvement (R2 = 0.53), and a ‘best practical fit’ model using baseline VO2 and sweat loss explains 49% of the variability of performance improvement (R2 = 0.49). Both models are statistically significant (p = 0.017; p = 0.023). The ‘best practical fit’ model interprets that those with low baseline VO2 but greater sweat loss in the first HRT demonstrated the best performance improvement, whereas those with high baseline VO2 and low sweat loss do not see much performance improvement. Chapter Three investigates the validity of the Kenzen™ wearable core temperature sensor. Ten participants engaged in two HRTs each (20 minutes at a steady state intensity immediately followed by a 30-kilometer time trial) whilst wearing a Kenzen™ device and a rectally inserted thermometer. Bland-Altman plots were used in conjunction with a direct comparison of the differences of the measures to determine validity. The Kenzen™ device is accurate between the range of 37-38 °C, but once core temperature measured rectally reaches 38.5 °C, the validity of the Kenzen™ device comes into question (when rectal temperature ≥ 39 °C, mean difference = 0.79 °C). The difference in measurements suggests that the Kenzen™ is not valid once moderate hyperthermia is reached (38.5 °C). Chapter Four presents a summary of the previous chapters. There is evidence to suggest that predicting variation in HA success is possible, potentially even more so with some refinement of the model through additional thermoregulatory measurement or protocol modification. The validity of wearable core temperature technology does not currently appear suitable in high-performance athletic environments where athletes are expected to reach core temperatures defined as hyperthermic, as most wearable sensors do not seem valid compared to gold standard measures. It is likely that the technology will advance as the popularity of such wearables increases.