This book provides a complete and up-to-date discussion on the theory, phenomenology, cosmology and astrophysics of massive neutrinos. Starting with a chapter on mathematical preliminaries that lead to the derivation of the standard model and its properties, it gives a discussion on the nature of Dirac and Majorana masses for neutrinos and the various extensions of the standard model (left-right, supersymmetric, etc.) that explain the small neutrino mass. Various phenomena related to neutrino mass such as rare decays of muons and kaons, double beta decay, and neutrinos in a medium, are discussed. The final chapters include discussions on solar and supernova neutrinos and neutrino cosmology. Extensive references to the existing literature are included.