2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/47705I 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.9 pages, no figure, latex, sprocl.sty, talk at the Workshop on B and Neutrino Physics, Allahabad, Jan 98High Energy Physics - Phenomenologytext