This paper is an attempt to trace, without going into details, the inherent logic and major lines of investigations, several results and also some difficulties involved in the study of medium-energy ion scattering by crystals. By “medium energies” is usually meant an energy region in which, strictly speaking, both mechanisms of energy losses (elastic and inelastic) should be taken into account. By tradition, as the limiting energy which demarcates the intervals with different dominant mechanisms of energy losses such an energy is usually accepted, which, being expressed in keV, is equal to atomic number of an incident ion independently of the target material‡ (the so called “simplified criterion of Seitz”).‡ Thus, the “medium energy” region is the energy interval in the neighbourhood of the above-mentioned limiting energy. In other words, this is the range in the order of a few keV to several tens of keV, depending upon the sort of ions used.