Theoretical investigations are reviewed concerning interactions of negative ion beams with rarified plasma oscillations directly behind the ion source, as well as with a dense plasma in a neutralizer. Effects of beam‐plasma instabilities on transportation of D−(H−) ions in an injector are studied. It is shown that nonlinear collective heating essentially influences phase characteristics of a slow ion beam during its formation and entering a rf accelerator. In the neutralizer, the beam emittance does not grow due to amplitude confinement of plasma oscillations at a low level as a result of capturing the beam electron component. A possibility of beam compression under the action of fast oscillating fields of electron surface plasma waves is considered.