Available for the first time in paperback, String Theory comprises two volumes which give an up-to-date, comprehensive and pedagogic account of the subject. Volume 1, An Introduction to the Bosonic String, gives a thorough introduction to the bosonic string, based on the Polyakov path integral and conformal field 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. Both volumes contain an annotated reference section, emphasizing references that will be useful to the student, as well as a detailed glossary of important terms and concepts. Many exercises are included which are intended to re-inforce the main points of the text and to bring in additional ideas. These volumes provide an essential text and reference for graduate students and researchers in theoretical physics, particle physics and relativity.