After the discovery of neutrino oscillations by SuperKamiokande three years ago the measurements of their masses and flavor mixing are under way using various neutrino sources and energies. High precision data on atmospheric neutrinos show that the dominant mixings occur between active species. They allow for statistically significant three-flavor analysis, which confirm the almost maximal mixing v(mu) <----> v(tau). Results of searches for tau neutrinos are consistent with this scenario. The K2K experiment, which uses the KEK accelerator as the neutrino source and the SuperKamiokande for their detection, is sensitive to the oscillations observed in the atmospheric neutrinos. The results obtained after one year of the run time provide a check of the oscillation parameters with the well determined beam properties using a set of near detectors and beam monitors. The solution of the solar neutrino puzzle in terms of v, oscillations has been recently much enhanced by exciting new results from the SNO Collaboration. Their comparison with SuperKamiokande results again excludes a sterile neutrino mixing as dominant suggesting instead a large v(e) mixing with other active neutrinos. This interpretation has been favored by recent SuperKamiokande spectral and angular measurements.

* Presented at the XXVII Mazurian Lake School of Physics, Krzyze, Poland, September 2-9, 2001.

The innermost secrets of the mysterious neutrino are being revealed in underground detectors. Recent data on atmospheric neutrinos, primarily from the Super-Kamiokande experiment, confirm the neutrino flavor mixing and non-zero masses. The high precision measurement of angular distribution allows to determine the value of Delta m(2) between 0.0013 eV(2) and 0.0054 eV(2) at 90% c.l. Studies of up-down asymmetries in different event samples indicate that nu(mu) <->nu(tau) oscillations are more likely explanation of the data than nu(mu) <-> nu(s). The deficit of the observed solar neutrino Aux compared to the predictions of the standard solar model, often interpreted by neutrino oscillations, is further studied in the SuperKamiokande detector. The energy spectrum is measured above 5.5 MeV for the Sun's positions above and below the horizon. A day-night effect is observed at a statistical significance of 2 sigma. The K2K (KEK to Kamioka) is the first long-baseline neutrino-oscillation experiment. During runs in 1999 the first 3 events were observed in the SuperKamiokande detector caused by neutrinos produced at the KEK accelerator at a distance of 250 km. The predicted number of events without any oscillations is 12.3(-1.9)(+1.7).

* Presented at the Cracow Epiphany Conference on Neutrinos in Physics and Astrophysics, Cracow, Poland, January 6-9, 2000.