  Intermetallic Heusler alloys are amongst the most attractive half-metallic
systems due to the high Curie temperatures and the structural similarity to the
binary semiconductors. In this review we present an overview of the basic
electronic and magnetic properties of both Heusler families: the so-called
half-Heusler alloys like NiMnSb and the the full-Heusler alloys like
Co$_2$MnGe. \textit{Ab-initio} results suggest that both the electronic and
magnetic properties in these compounds are intrinsically related to the
appearance of the minority-spin gap. The total spin magnetic moment $M_t$
scales linearly with the number of the valence electrons $Z_t$, such that
$M_t=Z_t-24$ for the full-Heusler and $M_t=Z_t-18$ for the half-Heusler alloys,
thus opening the way to engineer new half-metallic alloys with the desired
magnetic properties.
