The gut provides animals with the required energy for life. High selective pressures therefore act on the functionality of the gut. Despite these evolutionary pressures, very little changes in general gut anatomy are observed between taxa. In invertebrates and vertebrates three sections can be distinguished: 1) fore-gut for food-uptake and primary digestion, 2) midgut for further digestion and absorption, 3) hindgut for resorption of water and ions. Within this conserved anatomy, adaptation occurs by changes in absorptive area, retention time or volume fir storage or fermentation. In species with low energy diets, or high energy demands the digestive tracts is larger for better absorption and longer retention. In animals with high energy diets, or low energy demands the digestive tract is smaller. This change in gut-size is not only observed within evolutionary context, but also as phenotypic plasticity. When in some species the intestinal tract is temporarily in disuse, the intestinal tract becomes smaller and enlarges again when feeding is resumed. Although adaptation and plasticity of the gut seems to occur frequently, there are indications that it is limited by phyhgenetic or ontogentic constraints. Despite these constraints it is shown that trophic plasticity may play a major role in speciation.