This is a particularly exciting time for neutrino physics. Now providing the first experimental evidence for new physics beyond the Standard Model of particle physics, neutrino studies are leading that larger field in new direc­ tions. As a probe for discovery, neutrinos are unique among particles. Being leptons, neutrinos are as far as one knows true elementary particles, but in addition they are unencumbered by charge, and of course they do not have complicating strong interactions. Their unusually small mass also points to their novelty and possibly to their providing a window onto very high energy scales. The emphasis for neutrino physics at this time is on their masses and mixing with each other, as well as on their basic nature and their role in the Universe. While they have been important tools for studying particle properties, such as structure functions and the nature of the weak interaction, at present this is not the thrust of most research and hence is not covered in this book. Rather, the topics are those currently of most interest at the frontier of particle physics.