The dimensions of many creatures have been studied extensively, especially with respect to age, and none more so than fish, and that includes their gills. Since the primary function of the gill is the exchange of both respiratory gases and other solutes with the water inhabited by the fish, this chapter outlines how those dimensions determine the rate of solute exchange without invoking the very complex mathematics which tends to accompany texts on fluid dynamics. It discusses gas tension as the driving force and considers resistance to gas exchange and the particular point in the overall oxygen conduction line at which O2 molecules are transferred from aqueous respiratory medium to the blood. This is the step directly influenced by gill dimensions. The overall transport of respiratory gases with respect to metabolic demand of fish has been particularly well outlined by Hughes, who also makes interesting comparisons between water and air as the respiratory medium for a wide range of vertebrates.