Pigs, like all animals, have an ability to live in a variety of thermal environments. Although pigs can adapt so some degree to different thermal environments, there is a cost to production associated with keeping them outside their comfort zone. During cold stress, pigs consume more feed to maintain their core body temperature (the temperature of their vital internal organs). The extra feed consumed is not used for body weight gain. On the other end of the scale, heat stress will reduce feed intake in the pig; production is reduced in the case of the lactating sow because of the reduction in milk production. It is also observed that pigs heat stressed in the growing phase are fatter. There are many factors, which influence the thermal environment of the pig. Some of the more obvious are the temperature outside the barn and artificial heating in the barn. Others that are less obvious include stocking density in the pen, type of flooring and feed intake; however, there are many more. Most of these factors can be manipulated in some way by the producer or by the pig itself to reduce the effects of thermal stress. Although there are values in books, which indicate the optimal temperature ranges for pigs of different body weights and production phases, being able to recognize signs of cold or heat stress in your pigs is the best way to adjust the set point temperature and ventilation rate in your barn rooms.