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.

Invited talk at the XXth Internat. Conf. on Neutrino Physics and Astrophysics (Neutrino 2002), Munich, Germany, May 25-30, 2002. 12 pages, 18 figs, includes espcrc2.sty