In this chapter the physics of the interaction between metallic systems (bulk, surfaces and nanoparticles) and the electromagnetic radiation will be considered. The light-matter interaction will be analyzed for a plane-wave incident field as well as for a dipolar radiation, which represents the electromagnetic field emitted by molecules. We will start discussing the Maxwell’s equations, the dielectric function of metals and the localized surface plasmon resonances of metallic nanospheres. Then the Green’s function formalism and the electromagnetic scattering theory will be introduced. Finally the radiative and non-radiative decay rates of emitting molecules near metal surfaces and nanoparticles will be discussed.