Animals must maintain a steady physiological state, homeostasis, in response to a changing environment. An understanding of the stress response is essential if one is to appreciate the complex physiological mechanisms maintaining homeostasis in vertebrates. Moreover, a comprehensive understanding of the biology of stress is essential if we are to reduce stress in the care of any vertebrate species, induding those that are not domesticated and traditional laboratory animals.The goals of this chapter are to review our knowledge of the phenomenon of the physiological stress response in vertebrates generally, and then to describe in detail the limited information available on the biology of stress in reptiles. We shall not deal with psychic or emotional considerations of stress in reptiles. Specifically, we focus on the interactions between the physiological stress response and reproduction, immune function and intermediary metabolism. These three areas are critical to any husbandry and captive management programme. If not prevented or countermanded, the detrimental aspects of the stress response mounted by, for example,