Physical training and heat acclimation are both commonly adopted tactics to improve performance and/or tolerance times when individuals must compete or work in the heat. Potential benefits include: (i) improved aerobic fitness and thus a greater cardiovascular reserve (probably seen mainly after training); (ii) a lower resting body temperature that allows greater heat storage (probably seen mainly after acclimation); (iii) a decreased energy cost of a given intensity of exercise (seen after acclimation and also as the learning component of training); (iv) an enhanced sweating response at a given percentage of maximal effort (probably developed by both treatments); (v) a slower increase in body temperature owing to (iii) and/or (iv) [seen after both treatments]; (vi) a reduced cardiovascular stress because of changes in the autonomic nervous system (probably realised mainly by training), expansion of blood volume (seen after both treatments) and/or a decreased peripheral pooling of blood (probably found after both treatments); and (vii) improved subjective tolerance reflecting a decrease in the relative intensity of a given activity (probably seen mainly after training), a reduction in the physiological strain (found after both treatments) and/or habituation to heat-exercise stress (probably developed by both treatments). Factors affecting improvements in physiological and psychological responses to a given set of conditions include: (i) the individual's initial fitness and acclimatisation to heat; (ii) age, gender, hydration, sleep deprivation, circadian rhythms and in women the menstrual cycle; (iii) use of ergogenic aids such as fluid ingestion, carbohydrate and/or electrolyte replacement and blood doping; (iv) event or test conditions such as the mode of exercise, the severity of environmental heat stress and the type of clothing worn; and (v) treatment conditions such as the intensity, duration and frequency of exercise and/or heat exposure, the length of any rest intervals and cumulative depletion of body water and minerals.