This book reviews the current state of knowledge of neutrino masses and the related
question of neutrino oscillations. After an overview of the theory of neutrino masses and
mixings, detailed accounts are given of the laboratory limits on neutrino masses,
astrophysical and cosmological constraints on those masses, experimental results on
neutrino oscillations, the theoretical interpretation of those results, and theoretical
models of neutrino masses and mixings. The book concludes with an examination of the
potential of long-baseline experiments. This is an essential reference text for workers in
elementary-particle physics, nuclear physics, and astrophysics.