The monograph presents the results of theoretical and experimental studies of non-stationary processes of modification of metal surfaces by electric current, by switching it with a liquid-electrolyte electrode. This process is called plasma electrolyte treatment (PET). On the basis of the research carried out, complex hardening technologies and equipment have been created. Technologies allow oxidizing, alloying and changing the structure of local areas of the surface of products, which greatly increases their physical and mechanical properties without changing the structural state of the entire product. The complex of developed technologies multiplies: strength, wear resistance, fatigue strength of products under cyclic loads under conditions of friction and wear. Along with theoretical material, the monograph provides extensive information on the application and efficiency of the developed technologies in industry. The monograph is intended for engineering and technical workers of machine-building enterprises and institutes that specialize in the field of hardening processing of products.