  The primordial abundances of deuterium and of helium-3, produced during big
bang nucleosynthesis, depend sensitively on the baryon density. Thus, the
observed abundances of D and \he may provide useful ``baryometers'' provided
the evolution from primordial to present (or, presolar nebula) abundances is
understood. Inevitably, the derivation of primordial from observed abundances
requires the intervention of a model for galactic evolution and, so, the
inferred primordial abundances are, necessarily, model dependent. Here, an
analytic framework for the evolution of D and \he is presented which is
``generic'' in the sense that it should describe the results of any specific
galactic evolution model. The ``effective \he survival fraction'', $\Gamma _3$,
is the one free parameter which is model specific. Solar system and
interstellar data are used to infer upper and lower bounds to the primordial
deuterium mass fraction $(X_{2P})$ as a function of $\Gamma _3$ and, these
bounds are used to constrain the present baryon-to-photon ratio $(\eta)$ and
baryon density $(\Omega_B)$. For $\Gamma _3 \geq$ 1/4 it is found that (from D
and \he alone): $3.1 \leq \eta_{10} \leq 9.0$; $0.045 \leq \Omega_B h^2_{50}
\leq 0.133$ (where $H_0 = 50h_{50}$ km$\,$s$^{-1}$ Mpc$^{-1}$).
