This text attempts to present the whole range of quantum mechanics, from the fundamental assumptions to the experimental numbers. The author presents a unified theoretical formulation, in which every observable receives the emphasis it deserves for the particular problem being investigated, and includes examples from recent research. Earlier editions of this text have become a standard text and reference work for a generation of physicists. In this third edition, the author has made some alterations and corrections in the previous edition and has added two new chapters: one on quantal phase factors and their consequences (the "Berry phase") and another on the gauge theory of molecular physics.