Magnetism in III-V semiconductors is a comparatively new area of study: the first III-V based diluted magnetic semiconductors were produced in epitaxial films of (In,Mn)As grown on GaAs(100) substrates in 1989. An account is given of the magnetic behavior of these novel semi-magnetic alloys at various carrier concentrations. Finally, magneto-optical devices are presented in which the control of the carrier concentration by an electric field results in the manipulation of magnetic properties.