  We study the boundary states of D-branes wrapped around supersymmetric cycles
in a general Calabi-Yau manifold. In particular, we show how the geometric data
on the cycles are encoded in the boundary states. As an application, we analyze
how the mirror symmetry transforms D-branes, and we verify that it is
consistent with the conjectured periodicity and the monodromy of the
Ramond-Ramond field configuration on a Calabi-Yau manifold. This also enables
us to study open string worldsheet instanton corrections and relate them to
closed string instanton counting. The cases when the mirror symmetry is
realized as T-duality are also discussed.
