  In the last years we have learnt a lot about neutrino masses and mixings.
Neutrinos are not all massless but their masses are very small. Probably masses
are small because neutrinos are Majorana particles with masses inversely
proportional to the large scale M of lepton number (L) violation, which turns
out to be compatible with the GUT scale. We have understood that there is no
contradiction between large neutrino mixings and small quark mixings, even in
the context of GUTs and that neutrino masses fit well in the SUSY GUT picture.
Out of equilibrium decays with CP and L violation of heavy RH neutrinos can
produce a B-L asymmetry, then converted near the weak scale by instantons into
an amount of B asymmetry compatible with observations (baryogenesis via
leptogenesis). It appears that active neutrinos are not a significant component
of Dark Matter in the Universe. A long list of models have been formulated over
the years to understand neutrino masses and mixings. With the continuous
improvement of the data most of the models have been discarded by experiment.
The surviving models still span a wide range going from a maximum of symmetry,
with discrete non-abelian flavour groups, to the opposite extreme of anarchy.
