  Present and future observations of supernova relic neutrinos (SRNs), i.e., a
cosmological neutrino background from past core-collapse supernova explosions,
potentially give us useful information concerning various fields of
astrophysics, cosmology and particle physics. We review recent progress of
theoretical and observational studies of SRNs, particularly focusing on the
detectability and also on implications for cosmic star formation history and
neutrino physics.
