One of the eminent contributors to the field reviewed here launched, more than 10 years ago [1], a public appeal for closer cooperation between physicists and chemists on the problems raised by chemisorption and catalysis on solid surfaces. Enriched by many stimulating remarks, pointing to various directions of surface research, this appeal deserved more success; but solid state physicists, by and large, have been much too busy with clear-cut problems of their own. Besides, it is only very recently that semiconductor surface physics [2] has come of age.