Surface processes such as photodesorption, electron-stimulated desorption, photoluminescence, photocatalysis and photolytic decomposition rely on the population of excited electronic states in the adsorbate layer. A satisfactory description of both the excited and the ground state electronic structure of the adsorbate-substrate system is thus a necessary prerequisite for understanding these phenomena. This article attempts to explain simply some of the more important aspects of the adsorbate-substrate interaction and to describe how surface electronic structure can be probed spectroscopically.