To explain the interaction of EM radiation with materials at a level of the atomic structure of the material, we launch the Drude-Lorentz model, an upgrade of the Drude model from Chapter 3 on quantum mechanics. Using the Drude-Lorentz model, we review the distinctive optical properties of metals, insulators, and semiconductors. Besides reviewing the optoelectronic behavior of these bulk materials, we also deal with the interaction of electrons and photons with lower-dimensional materials. In the case of metals this brings us back to plasmonics, which we detail a bit further in this section. Then we review scaling of electronic and optical properties of semiconductors and compare semiconductor nanoparticles with metal nanoparticles.