Optical spectroscopy allows us to explore the structure of matter throughthe radiation-matter interaction processes (absorption, inelastic light scattering, emission/excitation, etc.). Given that optical spectroscopy utilizes UV-Vis-NIR photons to probe matter, this technique is important for studying matter under extreme conditions of pressure and temperature in diamond anvil cells (DACs) or other alternative transparent anvils such as sapphire (SAC) and moissanite (MAC) [1-14]. The huge advance of high-pressure techniques came with the design of the diamond anvil cell (DAC) that displays multidisciplinary character. The transparency range of diamonds in the UV-NIR region (0.07 eV to 5 eV) makes possible the adaptation of traditional spectroscopic techniques used in material characterization laboratories. Hence optical absorption, luminescence, excitation, lifetime or Raman techniques (see Chapter 9) can be systematically applied to work with microsamples and long working distances [15-17]. It is possible to investigate the changes in the electronic properties induced by local or crystal modifications of a material under high pressures without changing the chemical composition. We can determine different pressure effects like energy level shifts, local modifications or structural phase transitions and their impact on electronic and vibrational properties, changes in color due to pressure (piezochromism), high spin to low spin transitions, or excited-state crossovers.