This edition differs from the second chiefly in the addition of about tOO pages devoted to the quantum (or geometric, or Berry) phase, a subject that did not exist when this book was written. The changes in the remainder of the book consist of corrections of a small number of misprints. While it may seem that adding two chapters on the quantum phase is overemphasizing a currently fashionable subject, they actually complete the development of quantum theory as given in this book. We start with simple models, synthesizing them into complicated "molecules." With the new chap­ ters, we end with complicated "molecules," dividing them into simpler parts. This process of dividing a complex system into parts quite naturally gives rise to a gauge theory, of which the geometric phase is a manifestation - with consequences not only in theory, but observable in experiments. For this rea­ son, the geometric phase is not a mere fashion, but a discovery that will retain its importance forever and must be discussed in textbooks on quantum mechanics. I would like to acknowledge help and advice from Mark Loewe with the writing and also of the new part of the book. In addition, I would like to express my gratitude to J. Anandan, M. Berry, and C.A. Mead, who have read parts or all of the new material and have provided valuable advice.