Although some theoretical parameters of FOs may be obtained with photoemission studies, the STM studies of FOs are more common as they allow for direct observation of the phenomenon from the STM pictures or the dI/dV images. Using STM alone allows one to obtain a Fermi wavevector as half a radius of the first concentric ring around a point defect on the surface and the oscillation period as a spacing between the said concentric rings. Moreover, thanks to the new experimental technique called IETS (inelastic electron tunneling spectroscopy) 163 one can even obtain d 2 I/dV 2 images which can differentiate the type of contribution into the Friedel oscillations, just as in case of the inelastic phonon contribution. The FOs concept may have broad applications when describing systems with highly delocalized electrons. The response to the disturbing agent (e.g., point charge) depends on the degree of electron delocalization. We expect this fact to be explored in new delocalization descriptors with broad applicability. In principle, these can be simply based on charge analysis, however rigorous theoretical framework is still to be developed.