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 $(^3He/H)_P le 2.6times 10^{-5}$. 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.
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