Preface. 1. Introduction. 2. Conformal Field Theory for String Compactifications. 3. The Birth of Superstrings. Contributors: F. Gliozzi, M.B. Green, D.J. Gross, J.A. Harvey, E. Martinec, D. Olive, R. Rohm, J. Scherk, J.H. Schwarz. 4. Non-Supersymmetric Ten-Dimensional Strings. Contributors: L. Alvarez-Gaumé, L.J. Dixon, P. Ginsparg, J.A. Harvey, H. Kawai, W. Lerche, D. Lewellen, D. Lüst, G. Moore, A.N. Schellekens, N. Seiberg, S.H. Tye, C. Vafa, E. Witten. 5. Four-Dimensional Heterotic Strings.Contributors: I. Antoniadis, C.P. Bachas, L. Dixon, D. Gepner, J.A. Harvey, L.E. Ibañez, H. Kawai, C. Kounnas, W. Lerche, D.C. Lewellen, D. Lüst, M. Mueller, K.S. Narain, H.P. Nilles, F. Quevedo, M.H. Sarmadi, A.N. Schellekens, S.-H.H. Tye, C. Vafa, E. Witten. 6. Four-Dimensional Type-II Strings. Contributors: R. d'Auria, R. Bluhm, L. Castellani, L.J. Dixon, L. Dolan, F. Gliozzi, P. Goddard, V.S. Kaplunovsky, H. Kawai, W. Lerche, D.C. Lewellen, B.E.W. Nilsson, A.N. Schellekens, G. Sciuto, S.-H.H. Tye, C. Vafa.