  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, with a mass spectrum composed of two pairs of
neutrinos with close masses separated by the "LSND gap" of the order of 1 eV,
are compatible with the results of all neutrino oscillation experiments. In
these two schemes the probability of nu_e transitions into other states, the
probability of nu_mu->nu_e transitions and the size of CP violation effects in
nu_mu->nu_e transitions are suppressed in long-baseline experiments.
