  The Cosmic Microwave Background (CMB) is a relict of the early universe. Its
perfect 2.725K blackbody spectrum demonstrates that the universe underwent a
hot, ionized early phase; its anisotropy (about 80 \mu K rms) provides strong
evidence for the presence of photon-matter oscillations in the primeval plasma,
shaping the initial phase of the formation of structures; its polarization
state (about 3 \mu K rms), and in particular its rotational component (less
than 0.1 \mu K rms) might allow to study the inflation process in the very
early universe, and the physics of extremely high energies, impossible to reach
with accelerators. The CMB is observed by means of microwave and mm-wave
telescopes, and its measurements drove the development of ultra-sensitive
bolometric detectors, sophisticated modulators, and advanced cryogenic and
space technologies. Here we focus on the new frontiers of CMB research: the
precision measurements of its linear polarization state, at large and
intermediate angular scales, and the measurement of the inverse-Compton effect
of CMB photons crossing clusters of Galaxies. In this framework, we will
describe the formidable experimental challenges faced by ground-based,
near-space and space experiments, using large arrays of detectors. We will show
that sensitivity and mapping speed improvement obtained with these arrays must
be accompanied by a corresponding reduction of systematic effects (especially
for CMB polarimeters), and by improved knowledge of foreground emission, to
fully exploit the huge scientific potential of these missions.
