I will discuss a new class of multi-fermion F-terms that can appear in the effective actions of N=1 supersymmetric theories. Among other effects, such F-terms generate complex structure deformations of the moduli space of supersymmetric vacua of the theory. I will explain how these F-terms can be generated by instantons in N=1 supersymmetric string compactifications. Specifically, in the context of heterotic Calabi-Yau compactification, I will describe how worldsheet instantons can induce complex structure deformations of the moduli space. This talk is based on joint work with Edward Witten.