As discussed extensively in the current work, Friedel oscillations are a phenomenon arising from the Fermi liquid of electrons 159 formed in metallic systems. Unlike tight-binding models, which require electrons to be localized solely on atomic sites, models such as the nearly-free electron model allow for greater freedom. This results in the appearance of oscillations in response to an external perturbation of the system. We have presented many experimental examples where FOs appear on the STM images. Usually, they manifest themselves as concentric rings of electron density with the intensity sinusoidally oscillating with decreasing amplitude as their radii increase. Such an electron density pattern is similar to the mesomeric effect in chemistry. Unlike the latter, the Friedel oscillations travel in three dimensions and do not necessarily result in atom-wise charge alteration. The three-dimensional character of Friedel oscillations is also apparent from the STM pictures, showing the oscillatory pattern generated by the point defect below the materials' surface.