Called by some, "the theory of everything," superstring theory may lead to the unification of the two great theories of the twentieth century, general relativity and quantum field theory; it is one of the most active areas of research in theoretical physics. This course- tested comprehensive introductory graduate text stresses current areas of interest, including: string field theory, multi loops, Teichmueller spaces, conformal field theory, and four-dimensional strings. Prerequisites are an aquaintance with quantum mechanics and relativity. The second edition will be revised and updated throughout.