This chapter explains the physiology of oxygen transport from the ambient air that we breathe into our lungs all the way down to the muscle cells that use the oxygen to produce energy. Importantly, the chapter details how exposure to altitude stresses this oxygen transport system and drives adaptations that enhance the body's energy-producing mechanisms. It translates complex scientific concepts, like the oxygen pathway and the oxygen cascade, into practical terms to improve understanding of altitude training. Maximal aerobic capacity (VO2max) a key predictor of endurance, can be improved by physical training, but also with altitude exposure potentially offering an additional increase of up to 7%. Critically, performance improvements observed after altitude training sometimes occur without a corresponding increase in VO2max suggesting other beneficial adaptations in oxygen utilization occur with such training. The oxygen cascade describes the pressure-driven flow of oxygen, moving down a steep gradient from the high partial pressure of oxygen (PO2) in ambient air (159 mmHg at sea level) to the low PO2 in the mitochondria (10–20 mmHg at rest). At altitude, the lower barometric pressure (e.g., 580 mmHg at 2300 m) reduces the starting PO2 thus flattening the pressure gradient and challenging oxygen delivery. The body compensates by increasing ventilation and cardiac output. Training at altitude (typically 1500–2500 m) forces the body to adjust to this hypoxia, altering the oxygen pathway for improved efficiency upon returning to sea level.