This book provides an overview of traveling wave tubes in space applications. A traveling wave tube (TWT) is a type of vacuum tube used for high-power radio frequency amplification. This device uses an electron beam and is used in communication applications. Traveling wave tubes have many applications in space research. Traveling-wave tubes are also useful in ground-to-space transmissions; the device still has some use in telecommunication, including transponders for high-frequency military bands. Also, Traveling-wave tubes can be found in command-guidance systems for missiles and electronic countermeasures (ECM). The traveling-wave tubes can also be used as a radar transmitters.