Twenty-five years ago, Michael Green, John Schwarz, and Edward Witten wrote two volumes on string theory. Published during a period of rapid progress in this subject, these volumes were highly influential for a generation of students and researchers. Despite the immense progress that has been made in the field since then, the systematic exposition of the foundations of superstring theory presented in these volumes is just as relevant today as when first published. This two-volume set features a new Preface setting the work in context in light of recent advances. A self-contained introduction to superstrings, Volume 1 begins with an elementary treatment of the bosonic string, before describing the incorporation of additional degrees of freedom. Volume 2 is concerned with the evaluation of one-loop amplitudes, the study of anomalies and phenomenology. These books are invaluable for graduate students and researchers in general relativity and elementary particle theory, high energy physics and astrophysics, as well as mathematicians.