Gaugino Condensation, Loop Corrections, and S-duality Constraint

This talk is a brief review of gaugino condensation in superstring effective field theories and some related issues (such as renormalization of the gauge coupling in the effective supergravity theories and modular anomaly cancellation). As a specific example, we discuss a model containing perturbative (1-loop) corrections to the K\"ahler potential and approximate S-duality symmetry.