Nonperturbative effects on the string world sheet

We show that the space-time superpotential, which is known to receive no string theoretic corrections in finite orders in α′, is modified by world-sheet instantons. This has a number of interesting consequences for the physics of compactification schemes which lead to low-energy supersymmetry. We also present an argument that left-right symmetric Calabi-Yau spaces lead to conformally invariant sigma models both perturbatively and nonperturbatively.