2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48657Reconstruction of the neutrino mass spectrum and lepton mixing is one of the fundamental problems of particle physics. In this connection we consider two central topics: (i) the origin of large lepton mixing, (ii) possible existence of new (sterile) neutrino states. We discuss also possible relation between large mixing and existence of sterile neutrinos.32 pages, LaTeX, sprocl.sty, 9 figures. Invited talk given at the 5th International WEIN Symposium: A Conference on Physics Beyond the Standard Model (WEIN 98), Santa Fe, NM, June 14--21, 1998High Energy Physics - Phenomenologytext