In this talk I will outline the geometric ingredients of smooth compactifications of the heterotic string. This includes an SU(3) structure manifold (for example, a Calabi-Yau threefold) and a holomorphic, Mumford poly-stable vector bundle supported over it. Much recent work has attempted to explore the moduli spaces of such objects and the way that their properties determine the effective 4-dimensional physics. I will describe some of this progress, focusing in particular on fibration structures in Calabi-Yau manifolds and geometric transitions between topologically distinct Calabi-Yau threefolds.