The recent groundbreaking discovery of nzero neutri masses and oscillations has put the spotlight on massive neutris as one of the key windows on physics beyond the standard model as well as into the early universe. This third edition of the invaluable book Massive Neutris in Physics and Astrophysics is an introduction to the various issues related to the theory and phemelogy of massive neutris for the nexpert, providing at the same time a complete and up-to-date discussion on the latest results in the field for the active researcher. It is designed t merely to be a guide but also as a self-contained tool for research with all the necessary techniques and logics included. Specially emphasized are the various implications of neutri discoveries for the nature of new forces. Elementary discussions on topics such as grand unification, left-right symmetry and supersymmetry are presented. The most recent cosmological and astrophysical implications of massive neutris are also dealt with.