  We review here some of the checks of string-string duality between N=2, D=4
Heterotic and Type II models. The heterotic low energy field theory is
reproduced on the type II side. It is also shown to be a generalization in the
string context of the rigid $N=2$ Yang-Mills theory of Seiberg and Witten which
is exactly known. The non perturbative information of this rigid theory is then
recovered on the type II side. This talk is based on a work done in
collaboration with I. Antoniadis.
