  Extensive, accurate imaging of the Cosmic Background Radiation temperature
anisotropy at sub--degree angular resolution is widely recognized as one of the
most crucial goals for cosmology and astroparticle physics in the next decade.
We review the scientific case for such measurements in relation with sky
coverage, attainable sensitivity, confusing foreground radiation components,
and experimental strategies. Although ground--based and balloon--borne
experiments will provide valuable results, only a well--designed, far--Earth
orbit space mission covering a wide spectral range and a significant part of
all the sky will provide decisive answers on the mechanism of structure
formation.
