  I describe briefly the Cosmic Microwave Background (hereafter CMB) physics
which explains why high accuracy observations of its spatial structure are a
unique observational tool both for the determination of the global cosmological
parameters and to constrain observationally the physics of the early universe.
I also briefly survey the many experiments which have measured the anisotropies
of the CMB and led to crucial advances in observational Cosmology. The somewhat
frantic series of new results has recently culminated with the outcome of the
WMAP satellite which confirmed earlier results, set new standards of accuracy,
and suggested that the Universe may have reionised earlier than anticipated.
Many more CMB experiments are currently taking data or being planned, with the
Planck satellite on the 2007 Horizon poised to extract all the cosmological
information in the temperature anisotropies, and foray deeply into
polarisation.
