  We discuss the generation of superpotentials in four-dimensional, N = 1
supersymmetric field theories arising from type IIA D6-branes wrapped on
supersymmetric three-cycles of a Calabi-Yau threefold. In general, nontrivial
superpotentials arise from sums over disc instantons. We find several examples
of special Lagrangian three-cycles with nontrivial topology which are mirror to
obstructed rational curves, conclusively demonstrating the existence of such
instanton effects. In addition, we present explicit examples of disc instantons
ending on the relevant three-cycles. Finally, we give a preliminary
construction of a mirror map for the open string moduli, in a large-radius
limit of the type IIA compactification.
