  According to the standard models of particle physics and cosmology, there
should be a background of cosmic neutrinos in the present Universe, similar to
the cosmic microwave photon background. The weakness of the weak interactions
renders this neutrino background undetectable with current technology. The
cosmic neutrino background can, however, be probed indirectly through its
cosmological effects on big bang nucleosynthesis (BBN) and the cosmic microwave
background (CMB) radiation. In this BBN review, focused on neutrinos and, more
generally on dark radiation, the BBN constraints on the number of "equivalent
neutrinos" (dark radiation), on the baryon asymmetry (baryon density), and on a
possible lepton asymmetry (neutrino degeneracy) are reviewed and updated. The
BBN constraints on dark radiation and on the baryon density following from
considerations of the primordial abundances of deuterium and helium-4 are in
excellent agreement with the complementary results from the CMB, providing a
suggestive, but currently inconclusive, hint of the presence of dark radiation
and, they constrain any lepton asymmetry. For all the cases considered here
there is a "lithium problem": the BBN-predicted lithium abundance exceeds the
observationally inferred primordial value by a factor of ~3.
