2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48666The hierarchy $Δm^2_{atm} \gg Δm^2_\odot$ and the large $θ_{23}$ mixing angle, as suggested by neutrino oscillation experiments, can be accounted for by a variety of lepton flavour models. A dichotomy emerges: i) Models were all neutrino masses are bounded by $m_{atm}\equiv (Δm^2_{atm})^{1/2}\approx 0.03 eV$; ii) Models of quasi-degenerate neutrinos. It is shown how these different patterns of neutrino masses may arise from different lepton flavour symmetries. Physical implications are discussed in the various cases.8 pages, lecture at Erice 98High Energy Physics - Phenomenologytext