  I review the various mechanisms for obtaining nonzero neutrino masses. I
focus on the possibility of adding one light singlet neutrino to the three
known doublet neutrinos nu_e, nu_mu, and nu_tau to accommodate the totality of
neutrino-oscillation experiments. I review the various theoretical suggestions
for this singlet. I discuss also a scenario in which there are only the three
doublet neutrinos but a large anomalous interaction is assumed for nu_tau as an
attempt to explain all present data.
