A description of the methods of treating vellum developed and used by A. Cains in the workshop of Trinity College Library, Dublin. They are based on the premise that the method by which the skin was originally prepared should be borne in mind all the time treatment is in progress. Materials used in repair should also, if possible, be chemically similar to the skin. These include parchment size as an adhesive, goldbeater's skin, fish bladder membrane, vellum and parchment. The preparation of parchment size is described.