The spontaneous emission of an atom is not a property of the atom only; it also depends on the local optical surroundings. The simplest demonstration of this effect was provided by the early experiments of Drexhage, who studied the emission rate of luminescent europium ions close to a mirror. It was found that while the spectral distribution of the emission remained constant, the emission rate was dependent on the position of the Eu3+ ions relative to the mirror. This effect is due to interference of the optical modes incident to and reflected at the mirror. Since then, the modified spontaneous emission of atoms in cavities has been studied extensively. More recently, the control of spontaneous emission in solid-state systems has become of great interest because it enables the tailoring of the emission properties of optical materials. It was shown how the spontaneous-emission rate of optical probe ions or dyes inside dielectric films is modified by the presence of a dielectric interface, in a dielectric multilayer, or a microcavity. The dependence of the decay rate on the optical surroundings in these one-dimensional systems can be described in terms of Fermi's “golden rule,” which states that the decay rate is proportional to the local optical density of states (DOS). The spatial variation in the DOS is due to the interference of optical modes reflected and refracted at the dielectric interface(s).