An important and wide-reaching perturbation is through the action of electromagnetic fields, static or dynamic, homogeneous or space-dependent. In this chapter the response to an electromagnetic field is considered both for metallic and insulating (dielectric) systems. Both a macroscopic-like description, using Maxwell’s equations, and a simplified microscopic description, using simple classical equations of motion for a particle in some environment, are considered and connected. The concepts of Drude conductivity, plasma frequency, and time between collisions are discussed. The various types of modes are considered and the normal longitudinal modes are quantized (plasmons) and their dispersion relation is determined. The existence of screening in a conductor is discussed within the Thomas-Fermi approximation and a relation to the sound velocity is presented. Other excitations in solids are considered such as polaritons and excitons. In various problems discussed an analysis of the complex dielectric function and of the frequency-dependent complex conductivity is presented.