1. For introductions, see e.g. M. Green, J. Schwarz and E. Witten, Superstring Theory, Vols. I and II, Cambridge University Press, 1986; D. Liist and S. Theisen, Lectures on String Theory, Springer Lecture Notes in Physics, Vol. 346, 1990; M. Kaku, Introduction to Superstrings, Springer 1988
2. E. Verlinde, H.Verlinde, Lectures on String Perturbation Theory, in Superstrings 88, Trieste Spring School, Ed. Greene et al. World Scientific (1989) 189
3. C. Callan, D. Friedan, E. Martinec and M. Perry, Nucl. Phys. B262 (1985) 593, A. Sen, Phys. Rev. D32 (1985) 2102 and Phys. Rev. Lett. 55 (1985) 1846
4. D. Nemeschanski and A. Sen, Phys. Lett. 178B (1986) 365
5. For review, see A. Giveon, M. Porrati and E. Rabinovici, Target Space Duality in String Theory, hep-th 9401139, to be published in Phys. Rep.