  Recent atmospheric and solar neutrino experiments suggest that neutrinos have
small but nonzero masses. They further suggest that mass eigenvalues have
certain degree of hierarchical structures, and also some mixing angles are
near-maximal while the others are small. We first survey possible explanations
for the smallness of neutrino masses. We then discuss some models in which the
hierarchical pattern of neutrino masses and mixing angles arises as a
consequence of U(1) flavor symmetries which would explain also the hierarchical
quark and charged lepton masses.
