**6. Summary**

We have showed the importance of the mechanism of emission of dislocations from GBs, especially in case of NC materials where FR dislocation sources are difficult to operate. We also briefly mentioned the direct observation of GB dislocation mechanism by experimental methods. Of course, in even finer grains, dislocations have no role to play, except at the GBs. Therefore, although it appears that the dislocation pile-up model is still the most frequently favored model to explain the HP relationship, the other models have their own merit and advantages. It is especially clear that the Li mechanism plays an important role in the deformation studies of many types of materials, especially in nanomaterials. Furthermore, one should note that the HP relationships applies not only to yield stress, but also to flow and fracture stresses and even to fatigue and creep. Thus, it appears that the HP type size dependence of strength is a rather general result of natural dislocation behavior. Revealing various aspects of physics behind the HP law, especially in nanomaterials, remains an attractive and challenging type of research.

**Funding:** This research did not receive any external funding.

**Conflicts of Interest:** The authors declare that there is no conflict of Interest.
