The first experiments with relativistic magnetrons (PM), resulted in notable results, in the USA – Massachusetts Institute of Technology and the USSR - Institute of Applied Physics. Academy of Sciences of the USSR (Gorky), and the Nuclear Physics Research Institute at the Tomsk State University, hundreds of megawatts to several gigawatts with an efficiency of 10-30% were obtained. Relativistic high-frequency electronics has now become one of the fastest growing areas of scientific research. This reference is devoted to theoretical and experimental studies of relativistic magnetrons and is written by a leading expert who worked directly on these systems.