Covalent chromatography is an important, highly specific separation technique. In covalent chromatography, proteins and other ligands bearing accessible thiol groups form a mixed disulfide with the gel. The proteins are thus covalently linked to the stationary phase from which they can be subsequently eluted by addition of a reducing agent. Basic principles of the method are given together with the methods for the synthesis of gels. In addition, the paper describes several special applications of covalent chromatography.