  Constraints on big bang nucleosynthesis (BBN) and on cosmological parameters
from conflicting deuterium observations in different high red-shift QSO systems
are discussed. The high deuterium observations by Carswell et al., Songaila et
al., and Rugers & Hogan is consistent with 4He and 7Li observations and
Standard BBN (N_nu =3) and allows N_nu \leq 3.6 at 95% C.L., but is
inconsistent with local observations of D and 3He in the context of
conventional theories of stellar and Galactic evolution. In contrast, the low
deuterium observations by Tytler, Fan & Burles and Burles & Tytler are
consistent with the constraints from local Galactic observations, but require
N_nu = 1.9 +/- 0.3 at 68% C.L., excluding Standard BBN at 99.9% C.L., unless
the systematic uncertainties in the 4He observations have been underestimated
by a large amount. The high and low primordial deuterium abundances imply,
respectively, Omega_B h^2 = 0.005 - 0.01 and Omega_B h^2 = 0.02 - 0.03 at 95%
C.L. When combined with the high baryon fraction inferred from x-ray
observations of rich clusters, the corresponding total mass densities (for 50
\le H_0 \le 90) are Omega_M = 0.05 - 0.20 and Omega_M = 0.2 - 0.7, respectively
(95\% C.L.) The range of Omega_M corresponding to high D is in conflict with
dynamical constraints (Omega_M > 0.2 - 0.3) and with the shape parameter
constraint (Gamma = Omega_M h = 0.25 +/- 0.05) from large scale structure
formation in CDM and Lambda-CDM models.
