Physiological heat adaptations can be induced following repeated exposures to either hot artificial (i.e. heat acclimation) and natural (i.e. heat acclimatisation) environments that can improve aerobic performance in the heat and reduce the risk of exertional heat illness. The focus of this thesis was on the latter, with exercise heat acclimatisation referring to individuals, typically athletes and military personnel, relocating to a hotter climate to induce heat adaptations. The other more incidental form of heat acclimatisation is seasonal heat acclimatisation, where heat adaptations are induced from outdoor exposures during the summer months. In two systematic reviews of the literature and two separate laboratory-based studies, the magnitude of adaptation and the factors that influence the process of heat acclimatisation were investigated. In Chapter Three, a systematic search and meta-analysis of existing literature was conducted to quantify the effects of relocating to a hotter climate to undergo heat acclimatisation in athletes and military personnel. Whilst accounting for wet-bulb globe temperature (WBGT), daily training duration and protocol length, population estimates indicate a reduction in resting core temperature and heart rate of -0.19°C [90% credible interval (CrI): -0.41, 0.05; probability of direction = 91%] and -6 beats·min-1 [-16, 5; 83%], respectively. Furthermore, the rise in core temperature and heart rate during a heat response test were attenuated by -0.24°C [-0.67, 0.20; 85%] and -7 beats·min-1 [-18, 4; 87%], respectively. The findings of the review indicate that as athletes and military personnel relocate to a hotter climate, heat adaptations can be induced, with WBGT, daily training duration and protocol length all influencing the magnitude of adaptation. In Chapter Four, a systematic review of the literature was conducted to characterise seasonal heat acclimatisation during the summer months and identify key factors that influence the magnitude of adaptation. Following summer, 2 of the 16 studies reported reductions in resting core temperature (~0.16°C), 11 of 17 studies documented an increase in sweat rate (range: 0.03 – 0.53 L∙h-1), 2 of eight studies reported a decrease in heart rate during a heat response test (range: 3 – 8 beats∙min-1), and 6 of 8 studies observed reductions in sweat sodium concentration up to 59%. Of the studies that reported a statistically significant adaptation following summer, participants were exposed to a mean daytime WBGT of 25.2°C (range: 19.6 – 28.7°C). This review demonstrated that heat adaptations can be induced following summer in various climates. The magnitude of adaptation appeared dependent on several factors alongside the environmental characteristics, including the timing of environmental exposures during the day, and the duration and intensity of outdoor physical activity. The first laboratory-based study (Chapter Five) investigated the effect of the summer months on the process of seasonal heat acclimatisation in recreationally active adults (28 ± 8 yr, 54 ± 8 mL·kg-1·min-1) completing >5 h∙wk-1 of outdoor exercise by documenting the factors that influence the adaptive process. A heat response test was conducted before, during and after summer, consisting of 45 min of running at 60% V̇O2peak in 40°C and 30% relative humidity. Following summer, the rise in oesophageal temperature and mean skin temperature during the heat response test was lower by 0.12°C [-0.30, 0.06; 87%] and 0.43°C [-0.74, -0.10; 98%], respectively. Forearm local sweat rate increased (0.26 mg∙cm-2∙min-1 [0.15, 0.36; 100%]), whereas there was minimal evidence to suggest any changes in heart rate or whole-body sweat rate during the heat response test. As such, although partial heat adaptations were evident in the recreationally active adults, a combination of exercising later in the day and the prevailing environmental conditions (i.e. ~22.0°C daytime WBGT) may have hindered the further development of seasonal heat acclimatisation. The second laboratory-based study (Chapter Six) examined seasonal heat adaptations in active adolescents (15 ± 1 yr, 60 ± 10 mL·kg-1·min-1) following summer. Seasonal heat acclimatisation was evident as resting gastro-intestinal temperature and heart rate decreased by 0.19°C [-0.31, -0.07; 99%] and 7 beats·min-1 [-10, -3; 100%] post-summer, respectively. During the post-summer heat response test, an earlier onset of sweating was observed, in addition to an attenuated rise of gastro-intestinal temperature (0.22°C [-0.47, 0.04; 92%]). However, minimal changes were observed for resting haematological measures (i.e. haemoglobin mass, plasma volume and blood volume), or the additional sudomotor measures recorded during the heat response test (i.e. local and whole-body sweat rate). The results of this study suggest that heat acclimatisation was induced in active adolescents. Despite a temperate summer for the region (i.e. ~21.2°C daytime WBGT), the level of outdoor activity (~7 h∙wk-1) and the timing of the outdoor exposures (49% between 15:00 - 18:00) was sufficient to induce heat adaptations. The magnitude of the adaptation appears comparable to that reported previously in adults. This thesis examined the magnitude of adaptation that can be induced following both exercise heat acclimatisation (i.e. relocation) and seasonal heat acclimatisation. Seasonal heat acclimatisation was influenced by a series of environmental and physical activity characteristics, with the magnitude of adaptation indicative of partial heat adaptations. This thesis provides a contextualisation of seasonal heat acclimatisation in adults and includes the first investigation of seasonal heat acclimatisation in adolescents. The results of the thesis demonstrate that adolescents can attain heat adaptations during the summer months. However, future research is required to directly compare the adaptation kinetics of adults and children, as well as to determine whether younger children adapt to the heat of the summer months, along with the factors that influence the process of adaptation.