Relativistic electron beams are used successfully to generate electromagnetic waves. Both narrow-band and short-pulse THz generators of such type are described with examples of the free electron lasers (FELs) and prebunched-beam devices. FELs (see, e.g., [1]) convert the power of relativistic electron beams to narrow-band radiation. The main component of an FEL is a special spatially periodic magnetic system, called undulator (see, e.g., [2]). The undulator magnetic field curves electron trajectories; therefore, electrons emit narrow-band electromagnetic radiation in free space. The scheme of the FEL amplifier is shown in Fig. 1.