The interaction between the electromagnetic field and matter is an essential issue in modem physics. The physical concepts describing the field-matter interaction are of paramount importance, being at the origin of quantum mechanics and quantum field theories. On the other hand, the field-matter interaction is the main tool used nowadays to investigate the structure and properties of matter at the microscopic level. The subject of this book is the characterization of the solid-state through the interaction with the electromagnetic field. The main properties of different types of solids, such as dielectrics, semiconductors, disordered materials, impurified solids, low-dimensional structures, nanostructures, metals or superconductors, are revealed through their interaction with the electromagnetic field whose wavelength lies within the optical spectrum. Therefore, the book is focused on the investigation of the main properties of different types of solids that are obtained through light emission, absorption or scattering. There is already an immense literature dedicated to light-solid-state interactions or, in other words, the optical spectroscopy of solids, due to its major role in understanding the physics and properties of the solid-state. This literature is continuously growing and flourishing, due also to the paramount importance of solid-state devices in the development of any conceivable area of technology, including the booming information technology. However, this literature is mainly written emphasizing a certain type of materials (semiconductors, high-temperature superconductors, etc. ) or a certain type of optical spectroscopy method (Raman scattering, luminescence, etc. ).