  After an introduction to N=2 susy Yang-Mills theories, I review in some
detail, for the SU(2) gauge group, how the low-energy effective action is
obtained using duality and the constraints arising from the supersymmetry. Then
I discuss how knowledge of this action, duality and certain discrete symmetries
allow us to determine the spectra of stable BPS states at any point in moduli
space. This is done for gauge group SU(2), without and with fundamental matter
hypermultiplets which may have non-vanishing bare masses. In the latter case
non-trivial four-dimensional CFTs arise at Argyres-Douglas type points.
