I will begin with a review of perturbative heterotic vacua with N=2 spacetime supersymmetry. When such a vacuum is described by a non-linear sigma model, I will show that worldsheet supersymmetry requires the compactification geometry to be a principal torus bundle over a K3 manifold. In the second part of the talk, I will argue that a Calabi-Yau type IIA dual of such a vacuum should be based on a K3-fibered Calabi-Yau three-fold without a compatible elliptic fibration.