SEMINAR      Wednesday, 7 April 2004 - h. 11:00  Auditorium B. Touschek     G. Altarelli (CERN and Roma III) Neutrino Masses as a Probe of Grand Unification Abstract A natural explanation of the smallness of neutrino masses is that neutrinos are Majorana particles and their mass is inversely proportional to the large energy scale where lepton number interactions occur. The observed frequencies of neutrino oscillations point towards the Grand Unification Theory (GUT) scale. We discuss neutrino masses as probes of GUT’s and their implications for particle physics and cosmology. We review and compare theoretical models of neutrino masses which can reproduce the observed pattern of mixing angles.