Nonequilibrium cosmic neutrinos and nucleosynthesis

The neutrino role in primordial nucleosynthesis is reviewed. The importance of nonequilibrium effects is emphasized both for the standard massless and possibly massive neutrinos. The upper bound on tau neutrino mass is presented. A spatial variation of primordial abundances and a possibility of observing them by precise measurements of the CMB anisotropy are considered.The nucleosynthesis bounds on the parameters of neutrino oscillations into sterile neutrinos are discussed.