Optical spectroscopy is concerned with the measurement of the absorption, reflection and emission of light in the near-ultraviolet (~250 nm) through the mid-infrared (~3000 nm) portions of the spectrum. The human interface to the geological and mineralogical world is primarily visual. Optical spectroscopy is, in particular, well suited to investigating the origin of color in minerals. The reflection spectroscopy of minerals has been motivated to a large extent by interest in remote sensing. Emission spectra are usually studied in reference to luminescence phenomena. Studies of mineral color, metal ion site occupancy, oxidation states and concentrations have generally been done with absorption spectroscopy. This chapter concentrates on single crystal absorption spectroscopy. ; © 2014 Mineralogical Society of America.