Half-metallic Heusler alloys are amongst the most promising materials for
future magnetoelectronic applications. We review some recent results on the
electronic properties of these compounds. The origin of the gap in these
half-metallic alloys and its connection to the magnetic properties are well
understood. Changing the lattice parameter shifts slightly the Fermi level.
Spin-orbit coupling induces states within the gap but the alloys keep a very
high degree of spin-polarization at the Fermi level. Small degrees of doping
and disorder as well as defects with low formation energy have little effect on
the properties of the gap, while temperature effects can lead to a quick loss
of half-metallicity. Finally we discuss two special issues; the case of
quaternary Heusler alloys and the half-metallic ferrimagnets