  Radio interferometers are well suited to studies of both total intensity and
polarized intensity fluctuations of the cosmic microwave background radiation,
and they have been used successfully in measurements of both the primary and
secondary anisotropy. Recent observations with the Cosmic Background Imager
operating in the Chilean Andes, the Degree Angular Scale Interferometer
operating at the South Pole, and the Very Small Array operating in Tenerife
have probed the primary anisotropy over a wide range of angular scales. The
advantages of interferometers for microwave background observations of both
total intensity and polarized radiation are discussed, and the cosmological
results from these three instruments are presented. The results show that,
subject to a reasonable value for the Hubble constant, which is degenerate with
the geometry in closed models, the geometry of the Universe is flat to high
precision (~5%) and the primordial fluctuation spectrum is very close to the
scale-invariant Harrison-Zel'dovich spectrum. Both of these findings are
concordant with inflationary predictions. The results also show that the
baryonic matter content is consistent with that found from primordial
nucleosynthesis, while the cold dark matter component can account for no more
than ~40% of the energy density of the Universe. It is a requirement of these
observations, therefore, that ~60% of the energy content of the Universe is not
related to matter, either baryonic or nonbaryonic. This dark energy component
of the energy density is attributed to a nonzero cosmological constant.
