  We review the theory of the temperature anisotropy and polarization of the
cosmic microwave background (CMB) radiation, and describe what we have learned
from current CMB observations. In particular, we discuss how the CMB is being
used to provide precise measurements of the composition and geometry of the
observable universe, and to constrain the physics of the early universe. We
also briefly review the physics of the small-scale CMB fluctuations generated
during and after the epoch of reionization, and which are the target of a new
breed of arcminute-resolution instruments.
