Water channel proteins, aquaporins, are integral membrane proteins serving in the permeation of water and some other small molecules. Eleven isoforms of aquaporins have been identified from various tissues to date. They are expressed in tissue- and cell-specific manners, and are closely related to the specific functions of tissues and cells. Aquaporins are usually localized to the plasma membrane. Some isoforms are present in cytoplasmic compartments, and their translocation to the plasma membrane is crucial in the regulation of water transfer. This review focuses on the localization of aquaporins in mammalian tissues and discusses the physiological importance of water channels.