The general organization of this chapter begins with the anatomy of the gut, proceeding from anterior to posterior. Then the route is retraced to discuss the organ physiology of the digestive tract. A third pass through the tract discusses details of the enzymes. Then the typical methods used for studying digestion are described as a basis for the reader to make detailed comparisons among different methodologies and specific results or to perform his own experiments. Finally, some anatomical and functional comparisons are made for four species of fish with distinctly different feeding habits: carp (plant-oriented omnivore), catfish (animal-oriented omnivore), milkfish (specialized microplanktivore), and rainbow trout (carnivore). These comparisons are best made when accompanied by dissections of specimens of each species, although drawings are provided for readers having no access to the fish.