  In these notes, we emphasize the r\^ole of spontaneous broken global discrete
symmetries acting on the moduli space of $N=2$ susy Yang-Mills theories and
show how they can be used, together with the BPS condition, as a spectrum
generating symmetry. In particular, in the strong-coupling region, all BPS
states come in multiplets of this broken symmetry. This played a key r\^ole in
the determination of the strong-coupling spectra.
