General introduction; the evolution of vertebrate feeding mechanisms; adaptive interplay between feeding ecology and features of the digestive tract; factors affecting digestive efficiency; adjusting digestive capacity to change in amounts and types of foods eaten; geographic variation - the effect of differences in environmental variation on phenotypic plasticity of organ size and function; gastrointestinal responses to fasting in mammals - lessons from hibernators; cardiorespiratory responses to the elevated metabolic rate during digestion; determinants of intestinal brush border transport in ectotherms; control of gut motility and secretion in fasting and fed non-mammalian vertebrates; interplay between diet, microbes and immune defences of the gastrointestinal tract; effects of dietary fatty acids on physiological performance in fish; intestinal performance in developing fish larvae - proteins and amino acids; flexibility of organ size and function in response to feeding and fasting; the SDA response of ectothermic vertebrates - causes and determinants; the interplay between food's stoichiometry, digestion and metabolism - using stable isotopes to find out what animals eat; conclusions and perspectives.