Calabi-Yau threefolds crossing nodal singularities

We will review the conifold transition, which is a topological surgery for Calabi-Yau threefolds. The process involves a deformation of complex structure and a small resolution of nodal singularities. We discuss the geometrization of this process and its study by complex analytic and differential geometric methods. This is joint work with T. Collins and S.-T. Yau.