Solar neutrinos and lepton mixing

Solar neutrino problem and its solutions related to neutrino masses and mixing are discussed. There are two possible implications of the solutions to the Grand Unified Theories: quark-lepton symmetry (unification) and existence of the high (intermediate) mass scale. Structure of the intermediate scale is derived from the low energy neutrino data. However, reconciliation of different neutrino anomalies may require that solar neutrino deficit is explained by conversion of electron neutrinos into ultralight singlet fermion. The origin and properties of this fermion can be related to supersymmetry.