  The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to
measure the signature of a gravita-tional-wave background from inflation in the
polarization of the cosmic microwave background (CMB). CLASS is a
multi-frequency array of four telescopes operating from a high-altitude site in
the Atacama Desert in Chile. CLASS will survey 70\% of the sky in four
frequency bands centered at 38, 93, 148, and 217 GHz, which are chosen to
straddle the Galactic-foreground minimum while avoiding strong atmospheric
emission lines. This broad frequency coverage ensures that CLASS can
distinguish Galactic emission from the CMB. The sky fraction of the CLASS
survey will allow the full shape of the primordial B-mode power spectrum to be
characterized, including the signal from reionization at low $\ell$. Its unique
combination of large sky coverage, control of systematic errors, and high
sensitivity will allow CLASS to measure or place upper limits on the
tensor-to-scalar ratio at a level of $r=0.01$ and make a
cosmic-variance-limited measurement of the optical depth to the surface of last
scattering, $\tau$.
