In this second edition we have followed the framework of the original edition, changing or expanding the text only when warranted by new developments. The field of neutrino physics itself has been expanding at a rapid pace (as witnessed by our bibliography which has increased by 50% in just four years). Among the notable advances in the intervening years let us mention the opening of neutrino astronomy with the observation of the neutrino signal of SN1987A. The next supernova in, or near, our galaxy, is eagerly awaited by a number of “supernova watch” detectors. The detection of solar neutrinos has become an important subject of neutrino physics. Four detectors are running now, and several more are being built. The suppression of the solar neutrino flux (at least at higher energies) has been confirmed, and many physicists believe that solar neutrinos will represent the first observational manifestation of the neutrino mass and mixing. Double beta decay is another topic that has expanded considerably. The twoneutrino decay has been seen in the laboratory in three nuclei, thus it has become well established. The lifetime limits and the corresponding neutrino mass limits from the neutrinoless decay are continually improving. Better understanding of the nuclear matrix elements also puts double beta decay on a firmer footing.

In the broader context of particle physics as a whole the standard electroweak model appears to describe all observable phenomena. Yet, despite its success, it must eventually be replaced by a broader theory, with fewer parameters and deeper insight. The prospect of finding massive neutrinos remains one of the main possible openings to the world “beyond the standard model”.