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Diffuse reflectance spectroscopy (DRS) in the ultraviolet, visible and near-infrared region is a versatile spectroscopic technique, as both d-d and charge transfer transitions of supported TMI can be probed. One of the advantages of electronic spectroscopy is that the obtained information is directly chemical since the outer shell electrons of the TMI are probed and provide information about the oxidation state and coordination environment of TMI on surfaces. Furthermore, the DRS technique can be used under in situ conditions and is quantitative under well-defined conditions. The main disadvantage is that diffuse reflectance spectra are complex, and usually encompasses several broad and overlapping bands. To avoid biased spectral analysis, chemometrical techniques need to be employed. The chapter starts with a short overview of the principles of DRS spectroscopy. In the first two sections, theoretical and practical aspects of DRS, together with an introduction to crystal field, ligand field and molecular orbital theory are given. The third section will focus on selected examples in order to illustrate the potential and limitations of DRS spectroscopy for unravelling the chemistry of TMI on surfaces.