Recently, interest in and concern about animal well-being has been increasing because of changing views toward nonhuman animals, societal attitudes, legislation, and conduct of quality scientific research and testing. Animal well-being is a vague concept that can neither be viewed in a purely objective manner nor simply described, defined, or assessed. It is not a scientifically or technically precise state, but rather a multidimensional one. Factors such as animal needs and perspective, critical anthropomorphism, and human social and individual values are involved. There are limitations in determining the overall well-being of an animal and comparing well-being in disparate environments. Nevertheless, there is an extensive and ever-growing list of complex factors thought to affect homeostasis, sensitivity, interrelationships, and feedback control mechanisms. Research data from a variety of fields, such as animal biology and behavior, stress biology, and psychoneuroimmunology, increasingly support a holistic view of well-being. Existing data suggest an interactive system linking internal psychologic, neurologic, physiologic, immunologic, endocrine, and biochemical events with the external psychosocial and physical environment. An animal's state of well-being or homeostasis is determined by a multitude of external (psychosocial and physical stimuli) and internal (mental and biological responses) factors and interacting variables and by other aspects such as ethology, genetics, individual variation, social milieu, experience, learning, perception, coping style, and intensity, duration, and frequency of stimuli.