  Starting from the original Majorana's article of 1937, the see-saw mechanism
is illustrated, first for one and later for three neutrino generations, and
neutrinoless double beta decay is considered. Neutrino mixing and oscillations
in three flavors are described. The Yukawa couplings to the Higgs field of
quarks and leptons are considered, their transformation properties under the
corresponding flavor groups are spelled out and the principle of Minimal Flavor
Violation is illustrated, in connection with possible new physics beyond the
Standard Theory. The idea that the Yukawa couplings may be the vacuum
expectation value of some new fields is introduced and natural extrema of
potentials which are invariant under quark and lepton flavor groups are
characterized. A recent result indicating large mixing of almost degenerate
neutrinos is derived from the heavy lepton invariance under flavor ${\cal
O}(3)$.
