The various disorders resulting from heat stress can be understood and treated from an understanding of the underlying pathophysiology. Broadly speaking, they can arise as a result of excessive heat production from the body, or impairment of heat loss to the environment. The normal responses of the body to attempt to lose heat, peripheral vasodilation and sweating, are the root causes of the main disorders. In the tropics, the high environmental heat load leads to a spectrum of illnesses ranging from chronic exhaustion to acute and lethal heat stroke. Cooling and correction of fluid and electrolyte disturbances are the key to their treatment. In temperate climates, variants seen commonly occur in those imposing a metabolic heat load by endurance exercise. In these cases, there is some evidence that hyperventilation plays a role in their development, and this merits paper-bag rebreathing in addition to cooling and rehydration, during treatment. Those assisting at sporting events should attempt to stop competitors as soon as they show signs of heat exhaustion, and must be aware that almost all competitors will be hyperthermic towards the finish, and should be cooled and rehydrated as soon as possible.