This book discusses the basic physics of the generation of microwave and radiofrequency waves in the megawatt power range and the application of these ideas to a range of devices such as klystrons, gyrotrons, and free electron lasers. The book also covers the transmission of the power through waveguides and the problems associated with mode conversion in transmission lines. The application areas discussed are the heating and current drive in tokamaks and other devices for research into controlled nuclear fusion, multiple charged ion and soft x-ray sources, electron spin resonance spectroscopy, advanced materials processing, millimeter wave radar, and supercolliders.