Despite the hostile geographic and climatic conditions, millions of people live, work and reproduce at great altitudes in the mountainous regions of the Andes, the Himalayas, the Ethiopian highlands, the Rocky Mountains and the Swiss Alps. High altitude areas, defined as over 2,500 meters above sea level, are characterized by numerous atmospheric challenges, such as low temperatures, aridity, high levels of ultraviolet radiation, and, most important, decreased partial pressure of oxygen. In the high Andes it is possible to find permanent settlements located at more than 5,000 meters above sea level, where local populations live with half the atmospheric oxygen pressure than at sea level. How have entire populations managed to adapt to high altitude environments? What are the bodily mechanisms of adaptation to hypoxia? What are the physical and mental effects of prolonged exposure to low oxygen environments? How are disease and epidemiological patterns, mortality and morbidity rates affected by hypoxia? These are some questions medical practitioners and respiratory physiologists have been systematically trying to answer since the early twentieth century.