The transport of oxygen and carbon dioxide through contact lenses is important for maintaining normal corneal physiology. Gas permeation through a lens depends on the solubility and the diffusivity of the lens for that gas. Gases pass through spaces between the flexible polymer segments in plastics and rubbers. Gases diffuse through the water of hydration in hydrogel lenses, and gas permeation increases exponentially with hydration. The driving force behind gas permeation is the difference in partial pressure of the gas across the lens. The amount of gas passing through a given lens, in unit time, doubles by halving the lens thickness. Carbon dioxide permeation through any lens is several times greater than is oxygen permeation through that lens. Thus, it suffices to know the oxygen permeability in order to predict the physiological performance of a lens. The oxygen permeability coefficients of representative hard, hydrogel, and silicone rubber lenses are tabulated for easy reference.