A well-functioning digestive tract is key to a sustainable poultry production. Significant progress in our understanding of the digestive tract functionality has allowed for further improvements, and will continue to do so. Feed intake is a prerequisite for a high performance, where feed particle size is a key element. Although crop retention would potentially improve the digestion process, the practice of ad libitum feeding does not fully stimulate to its maximal use. The gizzard plays a very important role due to its very effective fine grinding and its role as a feed-flow regulator, but structural components need to be included in the diet. As material passes into the relatively short small intestine, it is remarkably quickly digested and absorbed. Fermentation in the caeca may contribute to some energy for the bird, but its major role seems to be water reabsorption, including from urine being refluxed into the caeca from the cloaca.