Abstract
Using the twistor approach to hypermultiplet moduli spaces, we derive theworldsheet, D(-1), and D1-instanton contributions to the generalized mirrormap, relating Type IIA and Type IIB string theory compactified on genericmirror Calabi-Yau threefolds. For this purpose, we provide a novel descriptionof the twistor space underlying the Type IIB hypermultiplet moduli space wherethe SL(2,Z)-action is found to be free from quantum corrections. The extent towhich instanton effects may resolve the perturbative singularities of themoduli space metric is discussed.