  Recent confrontations of the predictions of standard big bang nucleosynthesis
(SBBN) with the primordial abundances of the light nuclides inferred from
observational data reveal a conflict. Simply put, compared to theoretical
expectations the inferred primordial abundances of either deuterium or helium-4
(or both) are ``too small". Here I outline the ``tension" between D and $^4$He
in the context of SBBN. The incipient crisis for SBBN may be resolved by
observations of deuterium in nearly pristine environments such as the
high-redshift, low-metallicity QSO absorbers. At present the big bang
abundances of deuterium inferred from such data fall into two, apparently
mutually exclusive, groups. I describe the deuterium dichotomy and its
implications for SBBN as well as for cosmology in general.
