  These notes are based on lectures given by C. Callan and J. Harvey at the
1991 Trieste Spring School on String Theory and Quantum Gravity. The subject is
the construction of supersymmetric soliton solutions to superstring theory. A
brief review of solitons and instantons in supersymmetric theories is
presented. Yang-Mills instantons are then used to construct soliton solutions
to heterotic string theory of various types. The structure of these solutions
is discussed using low-energy field theory, sigma-model arguments, and in one
case an exact construction of the underlying superconformal field theory.
