Triathlon is a multievent sport (swimming, cycling, running). Long duration triathlons can induce physiological stress that can be modulated by environmental conditions. Certain factors promote performance, others limit it. A minimal level of maximal oxygen uptake is required, but it does not always determine the performance. For triathletes, the low hematocrit values do not reflect anemia and therefore do not limit performance. The appearance of clinical signs of dehydration and of digestive impairment may limit performance. The performance in swimming does not play the most important role in triathlon performance, but the physiological conditions in which the first transition is made can limit performance in the two following events; this is also the case for the second transition. Triathlon races cause muscle damage whose signs persist several days. Given the hormonal responses and the indices of muscle damage, it appears necessary to rest at least 5 days to avoid over-training. It is difficult to define precisely how much one should train for each of the three events. However, it can be concluded that triathlon training has to be taken as a whole.