Diffuse reflectance spectroscopy is a suitable technique for studying heterogeneous catalysts, as both d-d and charge transfer transitions of supported transition metal ions can be probed. Within the past several years, new developments have resulted in a more detailed understanding of the surface chemistry of supported metal oxide catalysts. In this review, the fundamental advances of the use of diffuse reflectance spectroscopy in the field of heterogeneous catalysis will be discussed. It will be shown that (1) oxidation states can be spectroscopically quantified in well-defined conditions; (2) chemometrical techniques are crucial for an unbiased analysis of the spectra; (3) in situ spectroscopy, in combination with experimental design and on line catalytic measurements, allows the development of relevant structure-activity relationships, and (4) the reduction kinetics of supported metal oxides can be evaluated.