  The concept of magnetic polaron is analysed and developed to elucidate the
nature of itinerant charge carrier states in magnetic semiconductors and
similar complex magnetic materials. By contrasting the scattering and bound
states of carriers within the $s-d$ exchange model, the nature of bound states
at finite temperatures is clarified. The free magnetic polaron at certain
conditions is realized as a bound state of the carrier (electron or hole) with
the spin wave. Quite generally, a self-consistent theory of a magnetic polaron
is formulated within a nonperturbative many-body approach, the Irreducible
Green Functions (IGF) method which is used to describe the quasiparticle
many-body dynamics at finite temperatures. Within the above many-body approach
we elaborate a self-consistent picture of dynamic behavior of two interacting
subsystems, the localized spins and the itinerant charge carriers. In
particular, we show that the relevant generalized mean fields emerges naturally
within our formalism. At the same time, the correct separation of elastic
scattering corrections permits one to consider the damping effects (inelastic
scattering corrections) in the unified and coherent fashion. The damping of
magnetic polaron state, which is quite different from the damping of the
scattering states, finds a natural interpretation within the present
self-consistent scheme.
