The fish liver plays an important role in vitellogenesis and, when compared with mammals, only a minor role in carbohydrate metabolism. On the other hand, the fish liver must be considered a target organ for many biological and environmental parameters that can alter liver structure and metabolism: food, pollutants, toxins, parasites, and microorganisms. Two major features of fish physiology must be taken into account when studying the liver: Fish are poikilothermal vertebrates, with substantial changes in metabolism related to temperature variations throughout the year. Fish spawn telolecithic eggs, rich in vitellus synthesized from precursor products through liver activity. The hepatic parenchyma of fish is very homogeneous and the hepatocytes are polygonal-shaped cells, often weakly basophilic, compared to those of mammals. Interestingly, studies of fish liver have shown that it appears well suited to serve as a model for analysis of the interactions between natural environmental changes and hepatic morphology.