  I review the interpretation of solar and atmospheric neutrino data in terms
neutrino oscillations and describe some ways to account for the required
neutrino masses and mixing angles from first principles, both within top-down
and bottom-up approaches. I also discuss non-oscillation phenomena such as
nu-less double beta which may probe the absolute scale of neutrino mass, and
also reveal its Majorana nature. I note that leptonic CP violation induced by
``Majorana'' phases drop from oscillations but play a role in the leptogenesis
scenario for the baryon asymmetry of the Universe. Direct tests of leptonic CP
violation in oscillation experiments, such as neutrino factories, will be a
tough challenge, due to the hierarchical neutrino mass splittings and the
smallness of theta13 indicated by reactors. The large solar mixing angle
theta12 offers a way to probe otherwise inaccessible features of supernova
physics. Finally, I note that in low-scale models of neutrino mass, one may
probe all mixing angles, including the atmospheric theta23, at high energy
accelerator experiments such as the LHC or NLC. A neat example is supersymmetry
with bilinear breaking of R parity, where the LSP decay branching ratios are
directly correlated to the neutrino mixing angles. I also discuss
non-oscillation solutions to the solar neutrino problem in terms of spin-flavor
precession and non-standard neutrino interactions, which will be crucially
tested at KamLAND.
