Neutrinos play a fundamental role in the latest particle physics theories, such as Grand Unified Theories, theories of supersymmetry, and superstring theory. Their mass yields an important boundary condition for grand unification models. They are the best candidates for dark matter in the universe, and their mass could determine its large scale structure and evolution. Neutrinos probe the interior of collapsing stars, and understanding them may lead to a solution of the solar neutrino problem. In ten chapters written by experts in each of these fields this book gives a comprehensive presentation of our current knowledge of the neutrino, of its role in nuclear particle and astrophysics theories, and of ongoing experimental efforts to learn more about its own nature. Graduate students and researchers in these fields will find this book a reliable advanced text and source of reference.