The groundbreaking discovery of nonzero neutrino masses and oscillations has put the spotlight on massive neutrinos as one of the key windows on physics beyond the standard model as well as into the early universe. The third edition of this text offers an introduction to the various issues related to the theory and phenomenology of massive neutrinos for the nonexpert, providing at the same time a discussion on results in the field for the active researcher. It is designed not 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 neutrino discoveries for the nature of new forces. Elementary discussions on topics such as grand unification, left-right symmetry and supersymmetry are presented. The cosmological and astrophysical implications of massive neutrinos are also dealt with.