The new results on the neutrino oscillations belong to the highlights of the particle and nuclear physics in the last few years. We tried to include these new developments in the present edition. Furthermore, we included a new section on the double beta decay. Of special interest is the possible neutrino­ less double beta decay. Its existence would require a non-trivial extension of the standard model of elementary particles. We have much appreciated the discussion and support of Gerry Garvey (Los Alamos) during the preparation of the revised version of the chapter on neu­ trino oscillations and the new chapter on double beta decay. We would like to thank Kunio Inoue (Sendai) for informing us about the newest results on neutrino mixing. We would like to thank Claudia Ries (Heidelberg) for carefully reading the manuscript and Jiirgen Sawinski (Heidelberg) for the excellent work he has done in formatting the book.