  A brief review of experimental and theoretical studies of half-metallic
ferromagnets (HMF) and spin gapless semiconductors (SGS) is given. The data on
resistivity and magnetoresistivity are presented. An important role of
non-quasiparticle states owing to electron-magnon scattering in transport
properties is discussed. The problem of low-temperature resistivity in HMF is
treated in terms of one-magnon and two-magnon scattering processes.
