Book excerpt: This report surveys the development of high-power cyclotron resonance masers (CRM) and gyrotrons in the Soviet Union, based on Soviet open-source literature. It deals with the nature and history of relativistic CRM devices; provides a sequence of the most important issues of gyrotron research and development as perceived by Soviet authors; and discusses individual Soviet research groups, the basic organizational units responsible for the CRM and gyrotron research and development. The study suggests, among other things, that high-power relativistic microwave electronics is one of the most successful areas of Soviet R&D. It has maintained a consistent record of significant achievements; it has managed to overcome the systemic weakness of the Soviet R&D system in being able to translate effectively the results of advanced research into production of practical equipment; and it has become the fastest growing area of application of pulsed power technology, which itself has been for many years the subject of priority development in the Soviet Union.