We review quantitative tests on the duality between the heterotic string on
T^2 and F-theory on K3. On the heterotic side, certain threshold corrections to
the effective action can be exactly computed at one-loop order, and the issue
is to reproduce these from geometric quantities pertaining to the K3 surface.
In doing so we learn about certain non-perturbative interactions of 7-branes.Comment: latex, 10 figs, contribution to the proceedings of Cargese 199
