To test the consistency of SBBN requires that we confront the
predictions with the primordial abundances of the light nuclides
which, however, are not observed but, rather, are inferred
from observations. The path from the observational data
to the primordial abundances is long and twisted and often fraught
with peril. In addition to the usual statistical and systematic
uncertainties, it is crucial to forge a connection from ``here and
now'' to ``there and then''; i.e., to relate the derived abundances to
their primordial values. It is indeed fortunate that each of the
key elements is observed in different astrophysical sites using very
different astronomical techniques. Also, the corrections for chemical
evolution differ among them and, even more important, they can be
minimized. For example, deuterium (and hydrogen) is mainly observed
in cool, neutral gas (so called H I regions) via UV absorption from
the atomic ground state (the Lyman series), while radio telescopes
allow helium-3 to be studied utilizing the analog of the hydrogen
21 cm line for singly-ionized 3He in regions of hot, ionized gas
(so called H II regions). Helium-4 is probed using the emission from
its optical recombination lines formed in H II regions. In contrast,
lithium is observed in the absorption spectra of warm, low-mass halo
stars. With such different sites, with the mix of absorption/emission,
and with the variety of telescopes and different detectors involved,
the possibility of correlated errors biasing the comparison with the
predictions of BBN is unlikely. This favorable situation extends to
the obligatory evolutionary corrections. For example, although until
recently observations of deuterium were limited to the solar system
and the Galaxy, mandating uncertain evolutionary corrections to infer
the pregalactic abundance, the Keck and Hubble Space telescopes have
begun to open the window to deuterium in high-redshift, low-metallicity,
nearly primordial regions
(Lyman-
clouds). Observations of
4He in chemically unevolved, low-metallicity (~ 1/50 of solar)
extragalactic H II regions permit the evolutionary correction to be
reduced to the level of the statistical uncertainties. The abundances
of lithium inferred from observations of the very metal-poor halo stars
(one-thousandth of solar abundance and even lower) require almost no
correction for chemical evolution. On the other hand, as noted earlier,
the status of helium-3 is in contrast to that of the other light elements.
For this reason, 3He will not be used quantitatively in this
article.
The currently very favorable observational and evolutionary situation for the nuclides produced during BBN is counterbalanced by the likely presence of systematic errors in the path from observations to primordial abundances. By their very nature, such errors are difficult - if not impossible - to quantify. In the key case of deuterium there is a very limited set of the most useful data. As a result, and although cosmological abundance determinations have taken their place in the current ``precision'' era of cosmology, it is far from clear that the present abundance determinations are truly ``accurate''. Thus, the usual caveat emptor applies to any conclusions drawn from the subsequent comparison between the predictions and the data. With this caution in mind the current status of the data will be surveyed in order to infer ``reasonable'' ranges for the primordial abundances of the key light elements.