The Advancea Kesearch Workshop on Superstrings was held on the campus of the University of Colorado at Boulder from July 27th through August 1, 1987. Since the work of Green and Schwartz in the summer of 1984, string theories have elicited tremendous amount of interest from both theoretical physicists and mathematicians. The objective of the Workshop was to bring together practitioners in the field to discuss the progress and problems, and possible directions of future research. There were ten talks of one hour each and twenty three talks of one-half hour each. The talks covered new formulations and technical developments. There were intense discussions both during and at the end of the lectures; 'further discussions continued during lunch and dinner. These proceedings contain all talks given at the Workshop except those by Victor Kac, Darwin Chang and Doron Gepner. The Workshop was sponsored by the North Atlantic Treaty Organization, which provided generous financial support enabling many young physicists from the U.S. and abroad to participate in the Workshop. Additional co-sponsors were the U.S. Department of Energy and the University of Colorado. The former offered further financial assistance and the latter furnished clerical and technical services and its campus facilities for the purpose of the organization and running of the Workshop. The International Organizing Committee consisted of John Ellis, Francois Englert, Peter G.O. Freund (co-director), K. T. Mahanthappa (co-director) and Abdus Salam.