Optical coatings are stratified media and, therefore, strongly anisotropic. The principal purpose of the stratification is to induce interference so as to yield the required spectral characteristics. At normal incidence the properties are largely insensitive to polarization, but at oblique incidence the sensitivity is significant. There may also be some birefringence in the material of the layers themselves, due to anisotropic strain or a directional microstructure, but, unless deliberately enhanced, their influence is normally small compared with that of the stratification. The effects are completely predictable and unambiguous, but, especially when coated components are combined into systems, they can be complicated and often unexpected. We normally divide optical coating materials into two major groups, metals and dielectrics. (Semiconductors are lumped with dielectrics.) These material classes behave differently. It is sometimes said that metals are insensitive to polarization. This is an oversimplification. Further, the surrounding media are usually dielectric, and so a coating incorporating a metal layer does exhibit a sensitivity to polarization, but of a somewhat different nature from that of a purely dielectric system. The polarization properties can be understood in terms of two linearly polarized eigenmodes defined in relation to the plane of incidence and designated as p- and s-polarization. Unfortunately, as is common in optics, there are several sign conventions, not always clearly defined when results are quoted.