A review is presented of biological issues and practical consequences of the effects of cold stress on ruminant animals. When animals are subjected to extreme cold stress, substantial dietary energy may be diverted from productive functions to the generation of body heat. Failure to produce sufficient heat can result in death. More often, however, cold stress leads to the development of secondary changes and possibly disease. With prolonged exposure to even mildly cold conditions, physiological adaptation occurs in animals resulting in increases in thermal insulation, appetite and basal metabolic intensity, as well as alterations in digestive functions. Much of the reduced productivity, and in particular the reduced nutritional efficiency, observed in ruminant production systems during the colder part of the year, can be accounted for by these adaptive changes.