  We consider the spectrum of BPS saturated states in $N = 2$ gauge theories in
four dimensions. This spectrum may be discontinuous across real codimension one
submanifolds of marginal stability in the moduli space of vacua. An example,
which can be treated with semiclassical methods in the weak coupling limit, is
the decay of quark-soliton bound states. For a quark and a soliton of
electric-magnetic charge vectors $Q$ and $Q^\prime$ respectively, we find that
as the manifold of marginal stability is crossed, the number of soliton states
changes by a factor of $2^{Q \cdot Q^\prime}$, where the dot denotes the
symplectic product.
