Fish represent the most common companion exotic animal in the United States and Canada in number of individuals and hold the third place after dogs and cats in terms of the number of households. Beyond companion animals, fish gastrointestinal diseases are particularly relevant for the aquaculture industry as feeding represents about 50% to 70% of the costs associated with fish production. Thus, nutrient malabsorption may have dramatic consequences both for the fish health and body condition score, but also for farmers. In this review, the authors discuss the anatomy, physiology, and pathology of the digestive system of fish.