Existing theories that describe the evolution of electrojet irregularities are critically reviewed, and the most likely mechanisms are considered. First, it is argued that the wave refraction theory is based on an invalid assumption and that the electrojet irregularities are not subject to refraction. Second, it is shown that there should be no significant wave-enhanced orbit diffusion, at least for two-strem irregularities because they are usually coherent. Third, it is pointed out that previous quasi-linear theories taking only the velocity retardation into account overlooked another important effect, that is, the density modification. A general quasi-linear theory including both effects is then presented for the two-stream and cross-field instabilities. It is concluded that the dominant stabilization mechanism of the two-stream instability must be electron drift retardation as previously proposed and that the dominant quasi-linear process for the cross-field instability in the electrojet is density flattening. Actually, not only density flattening but also mode coupling plays an important role in the evolution of the cross-field instability.