Solar Neutrinos and Lepton Mixing

With latest experimental data the solar neutrino problem enters new phasewhen crucial aspects of the problem can be formulated in an essentially (solar)model independent way. Original neutrino fluxes can be considered as freeparameters to be found from the solar neutrino experiments. Resonance flavourconversion gives the best fit of all experimental results. Already existingdata allow one to constraint both the neutrino parameters and the originalneutrino fluxes. The reconciliation of the solution of the solar neutrinoproblem with other neutrino mass hints (atmospheric neutrino problem, hot darkmatter etc.) may require the existence of new very light singlet fermion.Supersymmetry can provide a framework within which the desired properties ofsuch a light fermion follow naturally. The existence of the fermion can berelated to axion physics, mechanism of $\mu$-term generation etc..Comment: 9 pages, LaTeX, no figures. Talk given at the XXXth Rencontres de Moriond on Electroweak and Unified Theories, Les Arcs, Savoie - France, March 11 - 18, 199