High-intensity interval training (HIIT) refers to exercise that is characterized by relatively short bursts of vigorous activity, interspersed by periods of rest or low-intensity exercise for recovery. In untrained and recreationally active individuals, short-term HIIT is a potent stimulus to induce physiological remodeling similar to traditional endurance training despite a markedly lower total exercise volume and training time commitment. As little as six sessions of 'all-out' HIIT over 14 days, totaling ∼15 min of intense cycle exercise within total training time commitment of ∼2.5 h, is sufficient to enhance exercise capacity and improve skeletal muscle oxidative capacity. From an athletic standpoint, HIIT is also an effective strategy to improve performance when supplemented into the already high training volumes of well-trained endurance athletes, although the underlying mechanisms are likely different compared to less trained subjects. Most studies in this regard have examined the effect of replacing a portion (typically ∼15-25%) of base/normal training with HIIT (usually 2-3 sessions per week for 4-8 weeks). It has been proposed that a polarized approach to training, in which ∼75% of total training volume be performed at low intensities, with 10-15% performed at very high intensities may be the optimal training intensity distribution for elite athletes who compete in intense endurance events.