2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/40074We argue that the evidence for neutrino mass is quite compelling. This mass raises a number of questions, which we enumerate, about neutrinos. Then we focus on one of these questions---the issue of the possible neutrino mass spectra. In particular, we explain that one can have a four-neutrino spectrum which does not require significant sterile-neutrino involvement in either the atmospheric or solar neutrino oscillations.LaTeX, 6 pages, 4 embedded figuresHigh Energy Physics - Phenomenologytext