Characterizing/bounding geometries and geometric transitions in string compactifications

In this talk I will briefly review known bounds on the topology of CY manifolds and gauge fields over then. The field theory implications of these contraints can play an important role in characterizing both 4-dimensional N=2 and N=1 string vacua. I will also present new results on the geometry of Calabi-Yau conifold transitions as they arise in heterotic string compactifications. In particular, I will present particular classes of branes/bundles which can traverse the CY geometric transition in a controllable way.