  We discuss aspects of neutrino masses and lepton-number violation, in the
light of the observations of Super-Kamiokande. As a first step, we use the data
from various experiments, in order to obtain a phenomenological understanding
of neutrino mass textures. We then investigate how the required patterns of
neutrino masses and mixings are related to the flavour structure of the
underlying theory. In supersymmetric extensions of the Standard Model,
renormalisation group effects can have important implications: for small tanb,
bottom-tau unification indicates the presence of significant muon-tau flavour
mixing. The evolution of the neutrino mixing may be described by simple
semi-analytic expressions, which confirm that, for large tanb, very small
mixing at the GUT scale may be amplified to maximal mixing at low energies, and
vice versa. Passing to specific models, we first discuss the predictions for
neutrino masses in different GUT models (including superstring-embedded
solutions). Imposing the requirement for successful leptogenesis may give
additional constraints on the generic structure of the neutrino mass textures.
Finally, we discuss direct ways to look for lepton-number violation in
ultra-high energy neutrino interactions.
