Neutrino masses and mixing

Abstract

Status of determination of the neutrino masses and mixing is formulated and possible uncertainties especially due to presence of the sterile neutrinos are discussed. The data hint an existence of special ‘‘neutrino’’ symmetries. If not accidental these symmetries have profound implications and can substantially change the unification program. The key issue on the way to the underlying physics is relations between quarks and leptons. The approximate quark-lepton symmetry or universality can be reconciled with strongly different patterns of masses and mixings due to nearly singular character of the mass matrices or screening of the Dirac structures in the double see-saw mechanism.