If X and Y are a mirror pair of Calabi--Yau threefolds, mirror symmetry should extend to an isomorphism between the type IIA string theory compactified on X and the type IIB string theory compactified on Y , with all nonperturbative effects included. We study the implications which this proposal has for the structure of the semiclassical moduli spaces of the compactified type II theories. For the type IIB theory, the form taken by discrete shifts in the Ramond-Ramond scalars exhibits an unexpected dependence on the B-field. (Based on a talk at the Trieste Workshop on S-Duality and Mirror Symmetry.) Mirror Symmetry and the Type II String David R. Morrison a a Department of Mathematics, Box 90320, Duke University, Durham, NC 27708-0320, USA If X and Y are a mirror pair of Calabi--Yau threefolds, mirror symmetry should extend to an isomorphism between the type IIA string theory compactified on X and the type IIB string theory compactified on Y , with all nonperturbative effect...