Non-proportionality has an effect on all stage of the fatigue damage accumulation process. The mechanism of non-proportional fatigue is based mainly on the dislocation mechanism, which is evidenced in additional cyclic hardening. It is obvious, therefore, that it depends on many material properties. A basic micromechanic measure on which depends the material’s sensitivity to non-proportionality is stacking fault energy. It seems, however, that the dislocative mechanism of non-proportional loading has not been understood. It should be remembered that, according to some views, the effects of non-proportionality are not always the result of the intensification of dislocative processes. Materials that undergo a non-dislocative influence of non-proportionality, although sensitive to non-proportionality, do not manifest additional hardening.