  The evidence for non-vanishing neutrino masses from solar and atmospheric
neutrinos provides the first solid hint towards physics beyond the standard
model. A full reconstruction of the neutrino spectrum may well provide a key to
the theoretical structures underlying the standard model such as supersymmetry,
grand unification or extra space dimensions. In this article we discuss the
impact of absolute neutrinos masses on physics beyond the standard model. We
review the information obtained from neutrino oscillation data and discuss the
prospects of the crucial determination of the absolute neutrino mass scale, as
well as the intriguing connection with the Z-burst model for extreme-energy
cosmic rays.
