As we ascend to high altitude, air pressure falls and our bodies experience low oxygen availability – a condition known as hypoxia. In response, our heart rate and breathing rate increase – an attempt to maintain the supply of oxygen to our vital organs. Over time, levels of oxygen-carrying red cells increase in our blood. Meanwhile, the cells of our bodies, and the oxygen-consuming mitochondria within, re-wire their metabolism. This serves to decrease our bodies’ demand for oxygen and improve the efficiency at which we use this increasingly scarce but vital resource. Despite this process of acclimatisation, we remain limited by the low oxygen available to us, and this impacts our capacity to function, limiting our ability to exercise and think. Pregnancy at altitude poses a particular challenge, restricting growth of the developing fetus and potentially endangering the health of both mother and her offspring. In human populations that have spent thousands of years at altitude, including groups resident in the Himalayas and the Andes, there has been a selection of physiological traits, underpinned by genetic differences, which enable people to live, work and successfully reproduce. In this lecture, we will look at the responses of our bodies to altitude, and consider the different evolutionary strategies adopted by high altitude dwelling people. We will look at adaptations that support pregnancy at high altitude and will see how research into physiology at altitude is helping us to understand the condition of patients at sea level who experience hypoxia in common, but life-threatening contexts, such as complications of pregnancy or critical illness.