  Current observational data favor cosmological models which differ from the
standard model due to the presence of some form of dark energy and, perhaps, by
additional contributions to the more familiar dark matter. Primordial
nucleosynthesis provides a window on the very early evolution of the universe
and constraints from Big Bang Nucleosynthesis (BBN) can bound the parameters of
models for dark matter/energy at redshifts of order ten billion. The spectrum
of temperature fluctuations imprinted on the Cosmic Microwave Background (CMB)
radiation opens a completely different window on the universe at epochs from
redshifts of order ten thousand to nearly the present. The CMB anisotropy
spectrum provides constraints on new physics which are independent of, and
complementary to those from BBN. Here we consider three classes of models for
the dark matter/energy: extra particles which were relativistic during the
early evolution of the universe (X); Quintessence models involving a
minimally-coupled scalar field (Q); models with a non-minimally coupled scalar
field which modify the strength of gravity during the early evolution of the
universe (G). We constrain the parameters of these models using data from BBN
and the CMB and identify the allowed regions in their parameter spaces
consistent with the more demanding joint BBN and CMB constraints. For X and Q
such consistency is relatively easy to find; it is more difficult for the G
models with an inverse power law potential for the scalar field.
