  The CMB is perhaps the cleanest cosmological observable. Given a cosmology
model, the angular spectrum of the CMB can be computed to percent accuracy. On
the observational side, as far as we know, there is little that stands in the
way between an accurate measurement and a rigorous confrontation with theory.
In this article, we review the state of the data and indicate future
directions.
  The data clearly show a rise in the angular spectrum to a peak of roughly
$\delta T_l=(l(l+1)C_l/2\pi)^{1/2}\approx 85~\mu$K at $l\approx 200$ and a fall
at higher $l$. In particular, $\delta T_l$ at $l=400$ is significantly less
than at $l=200$. This is shown by a combined analysis of data sets and by the
TOCO data alone.
  In the simplest open models with $\Omega_m = 0.35$, one expects a peak in the
angular spectrum near $l=400$. For spatially flat models, a peak near $l=200$
is indicated and thus this model is preferred by the data. The combination of
this, along with the growing body of evidence that $\Omega_m \approx 0.3$,
suggests a cosmological constant is required. Further evidence for a
cosmological constant is provided by the height of the peak. This conclusion is
independent of the supernovae data.
