  The interpretation of the existing experimental evidences for oscillations of
neutrinos in schemes with three and four neutrino mixing is reviewed. Forms of
the lepton mixing matrix allowed by the neutrino oscillation data are
considered. The possible neutrino mass spectra compatible with the observations
are analyzed. The possibility to obtain information about the neutrino mass
spectrum from the future $^{3}$H $\beta-$decay and $(\beta \beta)_{0\nu}-$decay
experiments is also considered.
