  We provide the last missing piece of the complete non-perturbative
description of the low energy effective action emerging from Calabi-Yau
compactifications of type II string theory --- NS5-brane instanton corrections
to the hypermultiplet moduli space $M_H$. We find them using S-duality symmetry
of the type IIB formulation. The result is encoded in a set of holomorphic
functions on the twistor space of $M_H$ and includes all orders of the
instanton expansion.
