The main focus of the second volume of this fourth edition，as in the third，is on the two non-Abelian quantum gauge field theories of the Standard Model -that is，QCD and the electroweak theory of Glashow，Salam and Weinberg，We preserve the same division into four parts： non-Abelian symmetries，both global and local; QCD and the renormalization group; spontaneously broken symmetry; and weak interaction phenomenology and the electroweak theory.However，the book has always combined theoretical development with dis-cussion of relevant experimental results. And it is on the experimental side that most progress has been made in the ten years since the third edition appeared-first of all，in the study of CP violation in B-meson physics，and in neutrino oscillations. The inclusion of these results，and the increasing im-portance of the topics，have required some reorganization，and a new chapter （21） devoted wholly to them. We concentrate mainly on CP violation in B-meson decays，particularly on the determination of the angles of the unitarity triangle from B-meson oscillations.CP-violation in K-meson systems is also discussed. In the neutrino sector，we describe some of the principal experi-ments which have led to our current knowledge of the mass-squared differencesand the mixing angles.In discussing weak interaction phenomenology，we keep in view the possibility that neutrinos may turn out to be Majorana particles，an outcome for which we have prepared the reader in （new） chapters 4 and 7 of volume 1.