Sol-gel materials encompass a wide range of inorganic and organic/inorganic com­ posite materials which share a common preparation strategy. They are prepared via sol-gel processing involving the generation of colloidal suspensions (“sols”) which are subsequently converted to viscous gels and thence to solid materials. 1 This controlled method has many advantages, which led to its historical use before the underlying scientific principles were understood. In recent years increased under­ standing of these principles has led to a great increase of interest in the method, and to its application in the production of a wide variety of advanced materials. To appreciate these developments it is first necessary to consider some general features of the process.