SummaryThe triathlon is a 3-event endurance sport in which athletes compete sequentially in swimming, cycling and running. The primary determinant of success is the ability to sustain a high rate of energy expenditure for prolonged periods of time. Exercise training-induced physiological adaptations in virtually all systems of the body allow the athlete to accomplish this. Aerobic capacity (measured as maximal oxygen uptake, V̇O2max), economy of motion (submaximal V̇O2) and fractional utilisation of maximal capacity (%V̇O2max) reflect the integrated responses of these physiological adaptations.Numerous studies have reported relatively high mean V̇O2max) values for various groups of triathletes that are comparable to those reported for athletes in single-event endurance sports and clearly above those reported for untrained individuals. In shorter distance triathlons and in studies using recreational (rather than elite) triathletes, V̇O2maxis related to performance in the corresponding event of the triathlon (e.g. tethered swimming V̇O2maxwith swim time). In longer events and with more elite triathletes, V̇O2maxCorrelates less well with performance. The physiological adaptations that correspond to and facilitate improved V̇O2maxoccur centrally in the cardiovascular system, centred on increased maximal cardiac output, and peripherally in the metabolic systems, centred around increased arterio-venous O2(a-v O2) difference.While a high V̇O2maxin individuals is clearly of importance to triathlon performance, energy output must be sustained for long periods of time, making economy of motion also very important. Studies suggests that competitive swimmers have better swimming economy than triathletes. However, since many triathletes have previously been competitive swimmers this finding is questionable. The finding suggests that triathletes from nonswimming backgrounds would benefit from improving swimming technique rather than concentrating training workouts solely on distance. In cycling and running, comparison studies have not been done. Economy of motion in swimming, cycling and running have all been found to be correlated with comparable event performance. Training to improve swimming economy can be done without prior exercise, but training to improve cycling and running economy should take the multimode nature of a triathlon into consideration. That is, swimming should precede cycling economy training, and cycling should precede running economy training. Cardiovascular, metabolic and neuromuscular adaptations are the main physiological correlates of improved movement economy.Since exercise-induced stress on most physiological systems is based on relative, rather than absolute, exercise intensity, training and racing intensities are frequently quantified as a percentage of maximal capacity or %V̇O2max. Several studies of triathletes have examined the role of %V̇O2maxin triathlon performance, and have found relatively high correlations for each of the triathlon events with event-specific fractional utilisations. The %V̇O2maxaccessible is governed by several physiological systems, but most importantly by the lactate threshold. For the triathlete, lactate thresholds during swimming, cycling and running may all occur at different energy outputs, percentages of maximal capacity and heart rates. Lactate thresholds, therefore, must be measured independently in each of the 3 sports.In addition to V̇O2maxmovement economy and fractional utilisation of maximal capacity, considerations of energy balance, fluid and electrolyte balance as well as cardiac function over time are important for the success of the triathlete. The triathlete must maintain energy stores on a day-to-day basis during training and be able to generate energy at a high rate during the race. Likewise, fluid and electrolyte balance must be maintained daily as well as during the race. Continued and appropriate cardiovascular function is necessary for the transport of nutrients and oxygen, as well as for heat dissipation. Physiological considerations for triathlon training and racing are in many ways similar to those underlying successful performance of single-event endurance activities. However, because of the combination of skills and energy demands, the triathlon remains a unique physiological stress. Much remains to be investigated regarding the optimal physiological adaptations necessary for optimal and safe triathlon performance.