  We present new upper and lower bounds to the primordial abundances of
deuterium and helium-3 based on observational data from the solar system and
the interstellar medium. Independent of any model for the primordial production
of the elements we find (at the 95\% C.L.): $1.5 \times 10^{-5} \le (D/H)_P \le
10.0 \times 10^{-5}$ and $(^3He/H)_P \le 2.6\times 10^{-5}$. When combined with
the predictions of standard big bang nucleosynthesis, these constraints lead to
a 95\% C.L. bound on the primordial abundance of deuterium: $(D/H)_{best} =
(3.5^{+2.7}_{-1.8})\times 10^{-5}$. Measurements of deuterium absorption in the
spectra of high redshift QSOs will directly test this prediction. The
implications of this prediction for the primordial abundances of helium-4 and
lithium-7 are discussed, as well as those for the universal density of baryons.
