Offers an up-to-date discussion on the theory, phenomenology, cosmology and astrophysics of massive neutrons. It includes a chapter on mathematical preliminaries that lead to the derivation of the standard model and properties, and which gives a discussion on the nature Dirac and Majorana masses for neutrinos and the various extensions of the standard model 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. References to the existing literature are included.