Due to its fundamental importance, the movement of water across cell membranes has been an active area of research for more than 100 years. This subject is central to consideration of normal water metabolism by terrestrial animals, as well as derangements in overall water balance that are frequently encountered by nephrologists in the care of their patients. The objective of this review is to discuss the most basic aspects of cell membrane water permeability and provide a framework for these data in the context of the care of pediatric patients with renal disease. While the water permeability of most cell membranes can be accounted for by the diffusion of water across the lipid bilayer, other cells, including the red blood cell and certain epithelial cells that line the proximal and collecting tubules of the kidney and the urinary bladder of amphibians, possess specialized water channels. Water channels are composed of specialized proteins that create aqueous pores across cell membrane. Currently, there are active research efforts to isolate and characterize water channel proteins from these cell types. Data concerning the distribution, permeability and function of these various water channels will greatly enhance our knowledge of how water is transported across cell membranes.