  It is shown that at least four massive neutrinos are needed in order to
accommodate the evidences in favor of neutrino oscillations found in solar and
atmospheric neutrino experiments and in the LSND experiment. Among all
four-neutrino schemes, only two are compatible with the results of all neutrino
oscillation experiments. These two schemes have a mass spectrum composed of two
pairs of neutrinos with close masses separated by the "LSND gap" of the order
of 1 eV.
