  Forthcoming cosmic microwave background experiments (CMB) will provide
precise new tests of structure-formation theories. The geometry of the Universe
may be determined robustly, and the classical cosmological parameters, such as
the cosmological constant, baryon density, and Hubble constant, may be
determined as well. In addition, the ``inflationary observables,'' which
parameterize the shapes and amplitudes of the primordial spectra of density
perturbations and long-wavelength gravitational waves produced by inflation,
may also be measured and thus provide several new tests of inflation. Although
most attention has focussed on the more easily observed temperature
anisotropies, recent work has shown that the CMB polarization provides a wealth
of unique information that may be especially important for determination of the
inflationary observables. Secondary anisotropies at small angular scales
produced by re-scattering of photons from partial reionization may be used to
constrain the ionization history of the Universe.
