This volume is a compilation of lectures delivered at the TASI 2015 summer school, 'New Frontiers in Fields and Strings', held at the University of Colorado Boulder in June 2015. The school focused on topics in theoretical physics of interest to contemporary researchers in quantum field theory and string theory. The lectures are accessible to graduate students in the initial stages of their research careers.

String Theory, first published in 1998, comprises two volumes which provide a comprehensive and pedagogic account of the subject. Volume 2 begins with an introduction to supersymmetric string theories and presents the important advances of recent years. The first three chapters introduce the type I, type II, and heterototic superstring theories and their interactions. The next two chapters present important recent discoveries about strongly coupled strings, beginning with a detailed treatment of D-branes and their dynamics, and covering string duality, M-theory, and black hole entropy. The final chapters are concerned with four-dimensional string theories, showing how some of the simplest string models connect with previous ideas for unifying the Standard Model. They collect many important results on world-sheet and spacetime symmetries. An appendix summarizes the necessary background on fermions and supersymmetry. An essential text and reference for graduate students and researchers interested in superstring theory.

String Theory comprises two volumes which give a comprehensive and pedagogic account of the subject. Volume 1, first published in 1998, provides a thorough introduction to the bosonic string. The first four chapters introduce the central ideas of string theory, the tools of conformal field theory and of the Polyakov path integral, and the covariant quantization of the string. The next three chapters treat string interactions: the general formalism, and detailed treatments of the tree level and one loop amplitudes. Chapter eight covers toroidal compactification and many important aspects of string physics, such as T-duality and D-branes. Chapter nine treats higher-order amplitudes, including an analysis of their finiteness and unitarity, and various nonperturbative ideas. An appendix giving a short course on path integral methods is included. This is an essential text and reference for graduate students and researchers interested in modern superstring theory.

Volume 2: Superstring Theory and Beyond, begins with an introduction to supersymmetric string theories and goes on to a broad presentation of the important advances of recent years. The book first introduces the type I, type II, and heterotic superstring theories and their interactions. It then goes on to present important recent discoveries about strongly coupled strings, beginning with a detailed treatment of D-branes and their dynamics, and covering string duality, M-theory, and black hole entropy, and discusses many classic results in conformal field theory. The final four chapters are concerned with four-dimensional string theories, and have two goals: to show how some of the simplest string models connect with previous ideas for unifying the Standard Model; and to collect many important and beautiful general results on world-sheet and spacetime symmetries.

The two volumes that comprise String Theory provide an up-to-date, comprehensive account of string theory. Volume 1 provides a thorough introduction to the bosonic string, based on the Polyakov path integral and conformal field theory. The first four chapters introduce the central ideas of string theory, the tools of conformal field theory, the Polyakov path integral, and the covariant quantization of the string. The book then treats string interactions: the general formalism, and detailed treatments of the tree level and one loop amplitudes. Toroidal compactification and many important aspects of string physics, such as T-duality and D-branes are also covered, as are higher-order amplitudes, including an analysis of their finiteness and unitarity, and various nonperturbative ideas. The volume closes with an appendix giving a short course on path integral methods, followed by annotated references, and a detailed glossary.