This book grew out of a graduate course given in the Physics Department of the City College of New York for the first time during the 1976-1977 academic year and a series of lectures given at the Catholic University of Louvain, at Louvain-la-Neuve, Belgium during the Spring and Summer of 1977. I am indebted to Professor F. Brouillard and the DYMO group at that institution for the stimulation and hospitality provided during that period. In both cases, the lectures were at a level that assumed only a knowledge of elementary quantum mechanics of a typical first-year grad­ uate course. I have tried to continue that level of discussion in this book and to make it self-contained for any discussions that go beyond that level. In some sections of the book, the problems dealt with are too complicated to provide the entire description here. In that case, references to the original work are given.