Magnetic semiconductors have enjoyed a renaissance with the discovery by Komarov et al. in 1977 of very large enhancements of magneto-optical properties in Mn doped CdTe. Although these magnetic semiconductors are dilute, that is to say they are solid solutions of II-VI or IV-VI semiconductors in which the cations have been replaced (randomly) by divalent transition metal or rare earth ions, their physical properties may be understood, in general, with concepts developed for the concentrated systems, for example the europium chalogenides. It is the purpose of this paper to review some of these ideas and point out new developments which have broadened our understanding, particularly as regards magnetic polarons, the highly correlated ferromagnetic region surrounding trapped carriers in an otherwise paramagnetic (P.M.) or antiferromagnetic (A.F.) semiconducting hosts.