  The electronic structure of the rare earth nitrides is studied systematically
using the {\it ab-initio} self-interaction corrected local-spin-density
approximation (SIC-LSD). This approach allows both a localised description of
the rare earth $f-$electrons and an itinerant description of the valence
electrons. Localising different numbers of $f$-electrons on the rare earth atom
corresponds to different valencies, and the total energies can be compared,
providing a first-principles description of valence. CeN is found to be
tetravalent while the remaining rare earth nitrides are found to be trivalent.
We show that these materials have a broad range of electronic properties
including forming a new class of half-metallic magnets with high magnetic
moments and are strong candidates for applications in spintronic and
spin-filtering devices.
