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 and includes examples from recent research. Earlier editions of this text have become a standard text and reference work; it has been reprinted and translated. In this third edition, the author has made some minor corrections of 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.