  A comparison is given of the fundamental works of S. V. Vonsovsky on the
many-electron polar and s-d(f) exchange models with the subsequent development
of the theory of magnetism of transition and rare earth metals and their
compounds. Special attention is given to the derivation of different
many-electron models (by Heisenberg, Hubbard, Anderson) and to the
interrelation between them. Among the problems considered in this review, of
most importance are the many-electron approaches used in the description of
systems of d and f electrons, the atomic representation of X-operators, and the
problem of strong itinerant magnetism and formation of local moments. The
application of these concepts to highly correlated systems, in particular, to
the half-metallic ferromagnets and the Kondo lattices is discussed.
