Siliceous packing materials in which the surface hydroxyl group has been replaced by an organic group, have several unique and advantageous properties for chromatography. The several types of commercially available silicas useful for making chemically bonded stationary phases are described and compared. The three principal methods for derivatizing the silica surface-esterification with alcohols, treatment with organochlorosilanes, and organometallic reactions are reviewed. Pellicular materials are also considered. The mechanism of retention with chemically bonded phases is discussed, as well as column performance obtained with columns packed with these materials. Recent applications of chemically bonded phases are reviewed.