The monograph presents extensive theoretical and experimental material accumulated by the authors on the generation of relativistic electron beams with kiloamp currents and a duration of 50 ns to 10 μs, as well as the transportation of such beams in vacuum and plasma. The results of studies on the use of such beams for collisional heating of dense plasma and for generating millimeter waves in electrodynamic systems and terahertz radiation in plasma are discussed.