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The purpose of this paper is to discuss the properties of polaritons and the interactions in which they participate by methods that may be readily applied to a variety of physical situations. The discussion stresses the similarities between the properties of polaritons in dielectric crystals, where the dielectric tensor may exhibit strong frequency dependence, and in magnetic crystals, where the magnetic permeability tensor is strongly frequency dependent. We also examine the similarities and differences between ultrasonic waves and polaritons. This latter portion of the discussion is intended to be pedagogical in nature. T h e paper begins with a detailed discussion of the properties of bulk polaritons in an isotropic dielectric, with a single infrared active optical phonon. We introduce the phonon and photon strength functions, which provide a useful means of characterizing the polariton, and then examine polaritons in magnetic crystals, anisotropic dielectrics, materials in which spatial dispersion effects and absorption are important. We then review experimental methods of studying polaritons, with emphasis on Raman scattering, non-linear interactions between polaritons and recent experimental studies of non-linear optical phenomena where polaritons in the intermediate state play an important role. After this we review the properties of surface polaritons at the interfaces of the isotropic dielectric, anisotropic dielectrics and magnetic media. We also discuss experimental methods for the study of surface polaritons. The paper concludes with the comparison between the properties of coupled magnonphonon modes in magnetic crystals and polaritons.

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