String theory, once hailed as a potential "theory of everything," continues to progress at an astonishing rate, and this book brings the reader up to date with the latest developments and the most active areas of research in the field. New chapters discuss such topics as Seiberg-Witten theory, M theory and duality., and D-branes. The book includes a review of the fundamentals of string theory, making the presentation of the material self-contained while keeping overlap with the earlier book to a minimum. This book conveys the vitality of the current research and places readers at its forefront.