An increasingly large proportion of the world’s laying hens are kept in areas where heat stress is likely to be a major management factor at some stage during egg production. Basically the problem relates to birds not consuming enough feed at this time, although there are also some subtle changes in the bird’s metabolism that affect both production and shell quality. While all types of poultry thrive in warm environments during the first few weeks of life, normal growth and development of older birds is often adversely affected. Obviously, the bird’s requirements for supplemental heat declines with age, because insulating feathers quickly develop and surface area in relation to body size is reduced. Heat stress is often used to describe bird status in hot environments, although it is obvious that more than just temperature is involved. Because birds must use evaporative cooling (as panting) to lose heat at high temperatures, humidity of inhaled air becomes critical. Thus high temperature and humidity together are much more stressful to birds than high temperature alone. Other environmental factors such as air speed and air movement also become important. It is also becoming clear that adaptation to heat stress can markedly influence bird response. For example, laying birds can tolerate constant environmental temperatures of 35°C and perform reasonably well. On the other hand, most birds are stressed at 35°C when fluctuating day/night temperatures are involved. In the following discussion, it is assumed that fluctuating conditions exist, since these are more common and certainly more stressful to the bird.