V acuum-electronics technology is both old and new. Its legacy of useful devices is outstanding, and it is still evolving with inventions that take advantage of new materials, electromagnetic structures, and approaches to achieve record levels of performance and reliability. Although some view it as a stodgy remnant of an old technology, soon to be supplanted, it has been and, as new requirements emerge, will continue to be the enabling technology for entire classes of high-power high-frequency amplifiers with the most demanding specifications for use in both military and commercial systems.