MANY aspects of physiology are best understood in terms of bodily reactions to environmental stress, and temperature is one of the most often encountered stress factors in the environment. The responses to temperature can involve practically all of the organ systems of the body and it is for this reason that the study of the regulation of body temperatures represents one of the finest examples of complex reaction integrated by the nervous and endocrine systems, and hence of the principles of biological control. Thus, while thermoregulation offers an abundance of opportunities for the individual who likes to specialize in depth, it is an ideal type of physiology for those who prefer to think of the functioning of the body as whole. This book is written primarily for the undergraduate, but I hope also that some students may find time to read it, before embarking on a university course, as an introduction to some of the ideas that will be encountered in the more detailed study of the biological sciences, including medicine. I have tried to discuss the evidence for important ideas, since this is fundamental to the scientific method, and have been particularly concerned to avoid the use of the sort of technical jargon that gives a spurious impression of authority while in reality creating confusion out of what is in essence simple.