  The Cosmic Microwave Background (CMB) consists of photons that were last
created about 2 months after the Big Bang, and last scattered about 380,000
years after the Big Bang. The spectrum of the CMB is very close to a blackbody
at 2.725 K and upper limits on any deviations of the CMB from a blackbody place
strong constraints on energy transfer between the CMB and matter at all
redshifts less than 2,000,000. The CMB is very nearly isotropic, but a dipole
anisotropy of +/-3.346(17) mK shows that the Solar System barycenter is moving
at 368+/-2 km/sec relative to the observable Universe. The dipole corresponds
to a spherical harmonic index l=1. The higher indices l geq 2 indicate
intrinsic inhomogeneities in the Universe that existed at the time of last
scattering. While the photons have traveled freely only since the time of last
scattering, the inhomogeneities traced by the CMB photons have been in place
since the inflationary epoch only 10^{-35} sec after the Big Bang. These
intrinsic anisotropies are much smaller in amplitude than the dipole
anisotropy, with Delta T leq 100 microK. Electron scattering of the anisotropic
radiation field produces an anisotropic linear polarization in the CMB with
amplitudes less than 5 microK. Detailed studies of the angular power spectrum
of the temperature and linear polarization anisotropies have yielded precise
values for many cosmological parameters. This paper will discuss the techniques
necessary to measure signals that are 100 million times smaller than the
emission from the instrument and briefly describe results from experiments up
to WMAP.
