The ability of an animal to cope with new environments arises from its capacity to respond to environmental variables and maintain body equilibrium (homeostasis). Each compensating mechanism depends on, and is a part of, a physiological feedback process. The severity (intensity and duration) of an environmental change relative to the animal's capacity to respond determines the potential disruption to the animal's equilibrium and the resources that must be invested to regain homeostasis. However, an environmental change sufficient to seriously challenge one individual may be insufficient to produce a measurable response in another. The principles behind the responses occurring in animals as a consequence of a change in their physical environment are illustrated in this review by examples drawn from responses of animals to cold stress. Behavioral opportunities sometimes are constrained in farm animals, and internal metabolic responses tend to become more prominent in such situations. Furthermore, as a disturbing factor persists, the immediate defensive responses are replaced by longer-term and adaptive mechanisms that reduce the burden on the animal. As we gain greater understanding of the environment-animal interface and the sensitivity and response of animals to disruption, we will be better able to establish and maintain suitable environments for our farm animals.