In this book, the authors derive the theory of elastic depolarizing collisions and describe their importance in some nonlinear electromagnetic phenomena in gaseous media. The formation of photon echo and a description of its various types in gaseous media are then presented. The authors show that the characteristics of the corresponding signals depend essentially on elastic depolarizing collisions. They also consider the advantages of a new kind of photon echo spectroscopy: polarization photon echo-spectroscopy. A high-level, specialized treatment, Depolarizing Collisions in Nonlinear Electrodynamics will appeal to researchers and advanced graduates in nonlinear optics and quantum electronics.