This book brings together concepts and principles of physiology and evolutionary biology and blends them using optimality theory. The models therefore serve to integrate classical physiological information and to make predictions about which physiological strategies ought to be favoured in particular ecological circumstances. The approach ranges from a treatment of feeding strategies, including digestion, through costs of living and questions of growth, size and reproduction to general models of metabolism that begin to make sense of such fundamental physiological concepts as allometry. Examples are taken from existing physiological and ecological literature to illustrate arguments and test predictions. This new approach to physiological ecology shows not only that physiology has much to learn from ideas in evolutionary biology but that it can make a positive contribution to them.