Silica skins are surface films or layers, less than 1 mm thick, composed predominantly of silica, which generally form on stable quartzite and sandstone surfaces. They are important when they form on rock art surfaces because they protect the art from damage by graffiti and securely bind the ocher to the rock face. Occurrences of silica skins in Australia and in other countries are reviewed. The mineralogy, chemistry, and processes of formation of silica skins on rock substrates near prehistoric art sites have been investigated. If naturally occurring silica skins could be artificially produced and applied to unstable rock art surfaces, the art could be protected. Samples of silica skins were collected at various Australian locales, notably in Kakadu National Park. Solubility and other chemical properties of silica are reviewed. Different geochemical and textural types of silica skins have been found in a range of geological environments and climatic regimes and four types (IA, IB, 2A, 2B) are proposed and described. The role of substrate, water, and microorganisms (bacteria, algae, lichens) in the formation of the silica skins is discussed. Dating aspects of the skins are considered, including the possibility of radiocarbon dating of fossilized microorganic residues within the skins. Interventionist approaches to the protection of rock art, such as silicone driplines, should be considered only after evaluating the nature of the rock surface and the physiochemical and microorganic processes which interact on that surface.