The content of IHP Lecture notes 49 (final version) can be found in
DUALITY IN N=2 SUSY GAUGE THEORIES: LOW ENERGY EFFECTIVE ACTION AND BPS SPECTRA by A. Bilal
N=2 supersymmetry in 4 dimensions puts strong constraints on the form of any effective action. Following Seiberg and Witten, these constraints, together with electric-magnetic duality are shown to allow the determination of the exact low-energy effective action of N=2 susy gauge theories. The case of pure gauge theories and of extra hypermultiplets without and with bare masses are discussed. It is shown how the spectrum of stable BPS states is determined at every point in the moduli space.