The environmental factors of high ambient temperatures, high direct and indirect solar radiation, and humidity can stress animals. Although ruminants have thermoregulatory mechanisms, maintaining homeothermy under heat stress is difficult. There are negative consequences for productivity when animals experience hyperthermia. As body temperature rises, there is an adaptive depression of metabolic rate associated with reduced appetite. There are physiological (sweating, panting), hormonal (cortisol, thyroid gland activity) and behavioural responses that are activated in an attempt to maintain homeothermy. Heat stress may be aggravated by water deprivation, nutritional imbalance, and nutritional deficiency. This review also discusses how cattle are more sensitive to heat stress than sheep, and how animals of different productive stages are more susceptible to heat stress than animals at maintenance. Methods for alleviating heat stress under extensive conditions (such as providing shade shelter) are also discussed.