  The article reviews the conditions for lepton-flavor conservation in leptonic
interactions. The conditions are similar to those developed in the quark
sector. For neutrinos the mass eigenstates are unitary superpositions of flavor
states which together with small mass differences produce neutrino
oscillations. A new property for neutrinos is the fact that their masses are
very small suppressing their couplings to neutral scalar fields. The results
are demonstrated in several models.
