These notes are based on work done in collaboration with Frank Ferrari. We show how to determine the spectra of stable BPS states in N = 2 supersymmetric SU (2) Yang-Mills theories that are asymptotically free, i.e. without and with Nf = 1, 2, 3 quark hypermultiplets. In all cases: ∙ There is a curve of marginal stability diffeomorphic to a circle and going through all (finite) singular points of moduli space.∙ The BPS spectra are discontinuous across the curves. ∙ The strong-coupling spectra (inside the curves) contain only those BPS states that can become massless and are responsible for the singularities. Except for Nf = 3, they form a multiplet (with different masses) of the broken global discrete symmetry. ∙ All other semi-classical BPS states must and do decay consistently when crossing the curves. ∙ The weak-coupling, i.e. semi-classical BPS spectra, contain no magnetic charges larger than one for Nf = 0, 1, 2 and no magnetic charges larger than two for Nf = 3.