[en] Nonperturbative effects in the sigma model describing string propagation in curved space are considered. While the space-time superpotential is not renormalized in sigma-model perturbation theory, we show that it is changed by nonperturbative effects. Instantons can generate tadpoles and thus destabilize certain vacuum configurations, they can give masses to particles which are massless in perturbation theory and they can modify Yukawa couplings and high dimension operators in the superpotential. We consider the effects of perturbation theory around the instanton and multi-instanton configurations. Certain effects (pairing of particles in 27 and anti 2anti 7 and tadpoles in models with the spin connection equal to the gauge connection) vanish in the instanton approximation. We argue that this is an exact result. (orig.)