  The cosmic microwave background is now fulfilling its promise of determining
the basic cosmological parameters describing our Universe. Future study of the
microwave background will mostly be directed towards two basic questions: a
complete characterization of the initial perturbations, and probes of the
nonlinear evolution of structure in the Universe. The basic scientific issues
in both of these areas are reviewed here, along with possibilities for
addressing them with further microwave background measurements at higher
sensitivities and smaller angular scales. The proposed ACT experiment, which
will map 200 square degrees of sky at arcminute resolution and micro-Kelvin
sensitivity in three microwave frequency bands, is briefly described as an
example of rapidly advancing experimental technique.

Astrophysics