  We discuss the structure of heterotic/type II duality in four dimensions as a
consequence of string-string duality in six dimensions. We emphasize the new
features in four dimensions which go beyond the six dimensional vacuum
structure and pertain to the way particular K3 fibers can be embedded in
Calabi-Yau threefolds. Our focus is on hypersurfaces as well as complete
intersections of codimension two which arise via conifold transitions.
