  We study the F-terms in N=1 supersymmetric, d=4 gauge theories arising from
D(p+3)-branes wrapping supersymmetric p-cycles in a Calabi-Yau threefold. If p
is even the spectrum and superpotential for a single brane are determined by
purely classical ($\alpha^\prime \to 0$) considerations. If p=3,
superpotentials for massless modes are forbidden to all orders in
$\alpha^\prime$ and may only be generated by open string instantons. For this
latter case we find that such instanton effects are generically present. Mirror
symmetry relates even and odd p and thus perturbative and nonperturbative
superpotentials; we provide a preliminary discussion of a class of examples of
such mirror pairs.
