This article provides a general description of overall osmoregulatory mechanisms used by freshwater and marine teleost fish. It also has a general description of the characteristics of osmosis and water transport phenomena so that the reader is introduced to the language and concepts involved. There is then a more detailed description of the state-of-the-art concerning water transport mechanisms and more specifically the role of aquaporin water channels, in the gills, gastrointestinal tract, kidney, and urinary bladder, which together enable teleost fish to maintain water balance and achieve osmotic homeostasis.