HETEROTIC TYPE-II DUALITY IN D=4 AND STRING-STRING DUALITY

We discuss the structure of Heterotic/Type II duality in four dimensio ns as a consequence of string-string duality in six dimensions. We emp hasize the new features in four dimensions which go beyond the six dim ensional vacuum structure and pertain to the way particular K3 fibers can be embedded in Calabi-Yau threefolds. Our focus is on hypersurface s as well as complete intersections of codimension two which are conne cted via conifold transitions.