These review articles by outstanding specialists cover thepresent status ofthe observations of the spectrum and ofthe anisotropies of the cosmic microwave backgroundradiation. Experimental developments, data analysis andrelated theoretical aspects are also treated. The idea is toreview and discuss at a level accessible to non-specialisedastronomers and graduate students the most recentdevelopments in this field as well as the futureperspectives for astrophysics and cosmology in particular.