The use of spectroscopic techniques in the study of catalysts is rapidly increasing and has given a stimulus to research in catalysis. All techniques, however, are of restricted applicability; for example, ESR, NMR, reflection spectroscopy, and X-ray electron spectroscopy. Often a number of techniques have to be applied to give the full answer to a problem. The same is true for the technique under discussion in this paper. Luminescence spectroscopy can be helpful for a certain number of systems but is far from being generally applicable. It is, however, specific in the sense that it can solve certain problems, especially in oxidic catalysts, which are very difficult to solve with other techniques.