2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/42909We discuss two types of neutrino mass matrices which both give $θ_{23} = 45^\circ$, i.e., a maximal atmospheric mixing angle. We review three models, based on the seesaw mechanism and on simple extensions of the scalar sector of the Standard Model, where those mass matrices are obtained from symmetries.10 pages, plain LaTeX; talk presented by W. Grimus at the XXVII International Conference of Theoretical Physics, Ustron, Poland, 15-21 September 2003High Energy Physics - Phenomenologytext