This text describes the technique of optical spectroscopy applied to problems in condensed matter physics. It relates theoretical understanding to experimental measurement, including discussion of the optical spectroscopy of inorganic insulators, with many illustrative examples. Symmetry arguments are developed from a formal group theoretical basis and are frequently used, and a special effort is made to treat the subject of lattice vibrations and to show how these can affect the spectroscopic properties of solids. The elements of laser theory are developed, and the authors also explore the use of optically detected magnetic resonance techniques for the investigation of semiconducting materials.