From the Nobel Foundation:
After the discovery of asymptotic freedom and the emergence of QCD, he spent many years on the dynamics of gauge theories in the attempt to solve QCD. Much progress was made, but the goal of a controllable analytic solution of the theory was not realized. By the beginning of the 1980's, he along with many others had shifted his attention to more speculative physics - the challenge of unifying all the forces of nature and confronting quantum gravity. In the early 1980's, he began to work on string theory again and in 1984, together with a group of younger collaborators, discovered the heterotic string, which at the time seemed to offer the possibility of explaining the Standard Model from string theory. Since then he has been working largely on string theory. Although remarkable advances have been made, the potential of this theory to unify all the forces of nature and to make contact with experiment has not yet been realized.
Nobel Laureate, Physics Prize, 2004