This monograph begins by deriving the theory of elastic depolarizing collisions and then describes some nonlinear electromagnetic phenomena in gaseous media in which elasric depolarizing collisions are of primary importance. The formation of photon echo and a description of its various types in gaseous media are then included and it is shown that the characteristics of the corresponding signals depend essentially on the elastic depolarizing collisions. The significance of these collisions to corresponding signals and their characteristics is given and a new kind of photon echo spectroscopy (polarization photon echo-spectroscopy) is presented and its advantages considered. The final part describes the investigation of double-mode lasing of gas lasers and the coupling of strong and weak electromagnetic waves travelling through resonant gaseous media. This specialized and unique volume will be of interest to researchers and advanced graduates in the field of nonlinear optics and quantum electronics.