This chapter discusses the properties of III–V diluted magnetic semiconductors (DMS) and their theoretical understanding. Due to the semiconducting properties of DMS, the carrier concentration can be changed significantly by doping and even in situ by the application of a gate voltage, or by optical excitations. The equilibrium solubility of Mn in DMS with narrow to intermediate gaps lies one percent of cations, too low for ferromagnetic order to occur. The picture emerging for heavily doped DMS—holes in a weakly perturbed valence band coupled to local spins by an antiferromagnetic exchange interaction—is the Zener kinetic-exchange or pd-exchange model. DMS are highly disordered, since the magnetic dopants and the compensating defects are randomly distributed. Magnetic moments of defects have also been invoked to explain magnetic signals in other semiconductors, including samples without transition-metal or rare-earth dopants. The observed effective magnetic interactions are also highly anisotropic in real space.