The exploitation of the spectrum beyond 100 GHz is the solution for the full implementation of 5G and the development of 6G concepts. Low-power electronics is already available, but technology advancements are needed to overcome the increasing atmosphere and rain attenuation above 100 GHz, which presently limit the transmission distance.

New solid-state power amplifiers (SSPA) based on GaN or InP processes have been produced in the recent years, but the best output power achieved is far below the watt level needed for transmission by high modulation schemes over long range.

Vacuum electronic devices, namely, traveling-wave tubes (TWTs), offer more than one order of magnitude and more power than SSPA, over wide bandwidth, representing a promising solution for ultrahigh-capacity long links.

However, the short wavelength above 100 GHz poses substantial fabrication challenges, which require new technology approaches for high-volume production to bring TWTs into the wireless communication market. This chapter will explore the working mechanism, the potentiality, the features, the state of the art, and the possible deployment scenarios for millimeter-wave and sub-THz TWTs.