  When the perturbations forming the acoustic peaks of the cosmic microwave
background (CMB) reentered the horizon and interacted gravitationally with all
the matter, neutrinos presumably comprised 41% of the universe energy. CMB
experiments have reached a capacity to probe this background of relic
neutrinos. I review the neutrino impact on the CMB at the onset of the acoustic
oscillations. The discussion addresses the underlying physics, robustness or
degeneracy of the neutrino imprints with changes of free cosmological
parameters, and non-minimal models for the unseen radiation sector with
detectable signatures in CMB anisotropy and polarization.
