  We study four-dimensional low energy effective actions for conifold
transitions of Calabi-Yau spaces in the context of IIB supergravity. The
actions are constucted by examining the mass of D3-branes wrapped on
collapsing/expanding three-cycles. We then study the cosmology of the conifold
transition, including consequences for moduli stabilization, taking into
account the effect of the additional states which become light at the
transition. We find, the degree to which the additional states are excited is
essential for whether the transition is dynamically realized.
