Sub-degree anisotropy observations: Ground-based, balloon-borne and space experiments

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.