This chapter reviews the information published on fish digestive tract appendages (caeca) in divergent groups of fishes, and summarizes the findings of the investigations on their phylogeny, ontogeny, structure, and function. Teleost fishes include species with and without a stomach. A survey of both temperate and tropical fish revealed that more than 85% possess a stomach. Teleost fishes may be broadly divided into three feeding groups: omnivores, hervibores, and carnivores. Variation in stomach shape, size of intestine and number of caeca in the fish digestive tract has drawn the attention of many investigators. Two hypotheses regarding stomach-caeca-intestine relationship have been tested. First, since the stomach size and shape control the area around the gastrointestinal junction, and since the caeca are the gastrointestinal junctional appendages. Second, since the function of the caeca is to increase the surface area for the intestine, there should be some relationship between the number of caeca and size of intestine.