  For modeling the magnetic properties of concentrated and diluted magnetic
semiconductors, we use the Kondo-lattice model. The magnetic phase diagram is
derived by inspecting the static susceptibility of itinerant band electrons,
which are exchange coupled to localized magnetic moments. It turns out that
rather low band occupations favour a ferromagnetic ordering of the local moment
systems due to an indirect coupling mediated by a spin polarization of the
itinerant charge carriers. The disorder in diluted systems is treated by adding
a CPA-type concept to the theory. For almost all moment concentrations x,
ferromagnetism is possible, however, only for carrier concentrations n
distinctly smaller than x. The charge carrier compensation in real magnetic
semiconductors (in Ga_{1-x}Mn_{x}As by e.g. antisites) seems to be a necessary
condition for getting carrier induced ferromagnetism.
