Studies of the properties of plasma produced during the neutralization of the space charge of an intense ion beam are reviewed. Compensation of an ion beam by charges produced as a result of the ionization of a gas by the beam or by charges introduced from outside is considered. Particular attention is devoted to collective phenomena in ion-beam plasmas and, in particular, to nonlinear effects restricting the amplitude of the excited oscillations. It is shown that the propagation of compensated ion beams is influenced not only by dynamic decompensation but also by coulomb ion-electron scattering and by collective oscillations. All these processes must be taken into account when questions connected with the production of ``ultradense'' compensated beams are considered.