The results of nonlinear theory of interaction of modulated electron beams with semi-bounded plasma as well as with plasma waveguides and resonators are presented. Instability of a bounded electron beam in unbounded plasma is studies. Interaction of spiral electron beam with plasma waveguide is studied. Decay instability of electromagnetic field of an ultra-relativistic beam in plasma with respect to excitation of Langmuir and electromagnetic waves is studied. The process of excitation longitudinal oscillations in plasma with continuous wide spectrum by a relativistic electron beam in conditions when the beam density is comparable to that of the plasma is considered. Channels of beam energy loss in those conditions are studied and determinated. A theory of multi-wave interaction of high-energy relativistic electron beam with magnetized plasma waveguide is developed.