  A review of new developments in theoretical and experimental electronic
structure investigations of half-metallic ferromagnets (HMF) is presented.
Being semiconductors for one spin projection and metals for another ones, these
substances are promising magnetic materials for applications in spintronics
(i.e., spin-dependent electronics). Classification of HMF by the peculiarities
of their electronic structure and chemical bonding is discussed. Effects of
electron-magnon interaction in HMF and their manifestations in magnetic,
spectral, thermodynamic, and transport properties are considered. Especial
attention is paid to appearance of non-quasiparticle states in the energy gap,
which provide an instructive example of essentially many-body features in the
electronic structure. State-of-art electronic calculations for correlated
$d$-systems is discussed, and results for specific HMF (Heusler alloys,
zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.
