This chapter discusses life in hot environment, which are of two types hot-dry and warm-humid. Hot dry climate is found in deserts. Warm-humid climate is found in tropical forests. In hot dry climate the major challenge is to maintain sweat production. In warm-humid climate the major challenge is to promote evaporation of sweat. Heat accelerates chemical reactions and thereby stimulates cellular metabolism directly. There may be some initial hypoglycaemia during heat exposure. The urine shows significant antidiuretic activity after acute heat exposure suggesting that a humoral agent mediates the antidiuretic response. Relatively long-term exposure to hot environment leads to the development of physiological characteristics which make stay in that environment more convenient and comfortable. The adaptive changes during acclimatization are concerned primarily with sweat secretion. The almost universal physiological adaptations to hot climate are those concerning water conservation by minimising pulmonary, cutaneous, urinary and faecal water loss.