This book aims to provide a survey of the current state of research in the physics of neutrinos, which has undergone dramatic development during the last decade, in a form accessible to the nonspecialist and the graduate stu­ dent. The major issue during the last two decades has been the neutrino mass, whereas the interaction of neutrinos was well understood within the framework of the standard theory, which was established in the 1970s. In 1994, we published a textbook-format review article "Physics 01Neutrinos' in "Physics and Astrophysics 01 Neutrinos", in anticipation that the mass of the neutrino would be found in the near future. It was indeed found thanks to the Super-Kamiokande experiment four years after that book was written, and a flurry of activity followed in the phenomenology of the neutrino mass. From the theoretical point of view, the importance of this discovery sterns from the fact that it probably indicates the presence of a new energy scale beyond the standard theory. At the same time, the formalism for massive neutrinos and the techniques to find the neutrino mass have now realistic importance and have become the subject of standard physics. On the other hand, experiment revealed that the pattern of the neutrino mass and mixing among generations appears in a way different from what has been speculated on the basis of existing theoretical models.