String theory has advanced rapidly over the last 15 years and is increasingly seen as the best, and perhaps only, route to the complete unification of the four fundamental forces--the so-called theory of everything. This text provides, in two volumes, a thoroughly modern and comprehensive introduction to strings and superstrings, and brings the reader up to date on the latest developments in string duality, M-theory, D-branes, and the application of string theory to black hole quantum mechanics. The author is one of the world's top string theorists, and is also known as a clear and cogent writer. The two volumes are written at a level appropriate to graduate students in physics. This is the first major textbook on strings since the two volume work of Green, Schwartz and Witten 10 years ago (which sold extraordinarily well, reprinting within a month of publication). The main market for the book is theoretical physicists, particle physicists, astrophysicists, cosmologists and applied mathematicians. However, 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.