Traveling wave tubes play a crucial role in fields like communication and broadcasting. Planar slow wave structure traveling wave tubes can settle the power limitation caused by the size reduction in traditional cylindrical-beam traveling wave tubes. Therefore, there are increasingly more researchers contributing to the development of planar slow wave structure traveling wave tubes. As a competitive candidate for 5G/6G communication, planar slow wave structure traveling wave tubes are attracting a lot of attention. Yet there has not yet been a book which specifically introduces the key points in the development of such a device, this is that book. Readers will learn how planar slow wave structure traveling wave tubes work and how to design wideband, high efficiency, miniature millimeter wave and terahertz wave amplifiers. Part of IOP Series in Electromagnetics and Metamaterials.