String theory has advanced rapidly over the last fifteen years and is increasingly seen as the best, and perhaps only, route to the complete unification of the four fundamental forces - the so-called Otheory of everythingO. This text provides, in two volumes, a thoroughly modern and comprehensive introduction to strings and superstrings, and brings the reader up to date on all of the latest developments. The author is one of the world's top string theorists, and is also known as a clear and cogent writer. Ideal for graduate students and researchers in physics, this is the first major textbook on strings since the two volume work of Green, Schwartz and Witten ten years ago (which sold extraordinarily well, reprinting within a month of publication). The main markets are theoretical physics, particle physics, astrophysics, cosmology and applied maths. This is an accessible book on one of the most fundamental questions in physics by a top researcher. It has all the ingredients to be a best seller.