  These are notes from introductory lectures given at the Ecole Normale in
Paris and at the Strasbourg meeting dedicated to the memory of Claude Itzykson.
I review in considerable detail and in a hopefully pedagogical way the work of
Seiberg and Witten on $N=2$ supersymmetric $SU(2)$ gauge theory without extra
matter. This presentation basically follows their original work, except in the
last section where the low-energy effective action is obtained emphasizing more
the relation between monodromies and differential equations rather than using
elliptic curves.
