  Neutrino masses are usually described by adding to the Standard Model some
SU(2)-singlet fermions that have the Yukawa couplings, as well as some Majorana
mass terms. The number of such fields and the scales of their Majorana masses
are not known. Several independent observations point to the possibility that
some of these singlets may have masses well below the electroweak scale. A
sterile neutrino with mass of a few keV can account for cosmological dark
matter. The same particle would be emitted anisotropically from a cooling
neutron star born in a supernova explosion. This anisotropy can be large enough
to explain the observed velocities of pulsars. A lighter sterile neutrino, with
mass of the order of eV, is implied by the LSND results; it can have profound
implications for cosmology. We review the physics of sterile neutrinos and the
roles they may play in astrophysics and cosmology.
