Aquaporins (AQP) are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane. Structural analyses of the molecules have revealed the presence of a pore in the center of each aquaporin molecule. In mammalian cells, more than 10 isoforms (AQP0-AQP10) have been identified so far. They are differentially expressed in many types of cells and tissues in the body. Aquaporin 4 (AQP4) has been identified in a wide variety of tissues, including brain, lung, intestine, muscle, and kidney. It is highly expressed in the peri-vascular and subpial endfeet of glial cells, as well as in smaller amounts along the peri-neuronal membranes, and is presumed to play a vital role in maintaining homeostatic water balance across the blood–brain barrier. Furthermore, its presence as the primary water transporter in the human brain has led to considerable interest in better understanding its roles in human physiology and pathology.