  We discuss which information on neutrino masses and mixing can be obtained
from the results of all neutrino oscillation experiments in the cases of three
and four massive neutrinos. We show that in the three-neutrino case the
neutrino oscillation data are not compatible with a hierarchy of couplings. In
the case of four neutrinos, a hierarchy of masses is disfavored by the data and
only two schemes with two pairs of neutrinos with close masses separated by a
gap of the order of 1 eV can accommodate the results of all experiments.
