Instantons, Monopoles, Strings and Fivebranes

We discuss three classes of solitonic solutions in string theory: instantons, monopoles and string-like solitons. Instantons may provide a nonperturbative understanding of the vacuum structure of string theory, while monopoles may appear in string predictions for grand unification. The particular monopole solution shown has finite action as the result of the cancellation of gauge and gravitational singularities, a feature, which if it survives quantization, may yield insight into the structure of string theory as a finite theory of quantum gravity. In $D=10$, both monopoles and instantons possess fivebrane structure. The string-like solitons represent extended states of fundamental strings.( Talk given at INFN Eloisatron Project: 26th Workshop: ``From Superstrings to Supergravity'', Erice, Italy, Dec. 5-12, 1992.)