Neutris play a decisive part in nuclear and elementary particle physics, as well as in astrophysics and cosmology. Because they interact so weakly with matter, some of their basic properties, such as mass charge conjugation symmetry, are largely unkwn. These subjects are considered in detail by authors, who also discuss such topics as neutri mixing, neutri decay, neutri oscillations, double beta decay and related ideas. Physical concepts are stressed, and both theoretical methods and experimental techniques are presented. This second edition contains an expanded coverage of new experimental results and recent theoretical advances. In the intervening years since the first edition, many then unresolved problems such as tritium beta decay and reactor neutri oscillations have been clarified. This edition also gives expanded coverage of solar and superva neutris.