  Limits can be placed on nonstandard neutrino physics when big bang
nucleosynthesis (BBN) calculations employing standard neutrino physics agree
with the observationally inferred primordial abundances of deuterium (D),
$^3$He, $^4$He, and $^7$Li. These constraints depend most sensitively on the
abundances of D and $^4$He. New observational determinations of the primordial
D and/or $^4$He abundances could force revisions in BBN constraints on
nonstandard neutrino physics.
