This chapter discusses life at high altitude, where the primary problem at high altitude is low atmospheric pressure, although a fall in environmental temperature is also associated with it. The physiological effects of high altitude may be arbitrarily divided into acute responses and acclimatization. The acute effect of high altitude is a reduction in work capacity. At higher altitudes, there may be loss of judgement, twitchings, convulsions and finally unconsciousness. Carbon dioxide washout tends to produce respiratory alkalosis. The partial pressure of oxygen and nitrogen is lower in the respiratory passages than in the atmosphere. Greater the tidal volume, greater will be the fraction of alveolar air replaced by atmospheric air with each breath. Water vapour pressure forms part of the total pressure of gases in the respiratory passages. Increase in haemoglobin increases the oxygen carrying capacity of blood. Hypoxia is a potent stimulus for erythropoiesis.