Lectures on string theory

The book presents an extended version of the lectures on string theory which were given by the authors in 1987-1988 at the Max-Planck Institut für Physik und Astrophysik in Munich. It can be considered as an introduction to the current string theory which could achieve a unification of gravitation with all other interactions in the frames of quantized field theory. The present lecture notes are intended mainly to provide some of the tools and first of all the conformal invariance of the field theory which are necessary for the construction of four- dimensional (heterotic) string theories. One of such constructions, namely the covariant lattice four-dimensional theory, is discussed in detail. \par Contents: the classical and quantized bosonic string; introduction to conformal field theory; reparametrization ghosts and BRST quantization; global aspects of string perturbation theory and Riemann surfaces; the classical and quantized closed fermionic string; spin structures and superstring partition function; toroidal compactification of the closed bosonic string - 10-dimensional heterotic string; lattices and Kac-Moody algebras in conformal field theory; superconformal field theory; bosonization of the fermionic string - covariant lattices; heterotic strings in ten and four dimensions; low energy field theory