  We continue our study of compactifications of F-theory on Calabi--Yau
threefolds. We gain more insight into F-theory duals of heterotic strings and
provide a recipe for building F-theory duals for arbitrary heterotic
compactifications on elliptically fibered manifolds. As a byproduct we find
that string/string duality in six dimensions gets mapped to fiber/base exchange
in F-theory. We also construct a number of new $N=1$, $d=6$ examples of
F-theory vacua and study transitions among them. We find that some of these
transition points correspond upon further compactification to 4 dimensions to
transitions through analogues of Argyres--Douglas points of $N=2$ moduli. A key
idea in these transitions is the notion of classifying $(0,4)$ fivebranes of
heterotic strings.

High Energy Physics - Theory