The spectrum of stable electrically and magnetically charged supersymmetric
particles can change discontinuously as one changes the vacuum on the Coulomb
branch of gauge theories with extended supersymmetry in four dimensions. We
show that this decay process can be understood and is well described by
semiclassical field configurations purely in terms of the low energy effective
action on the Coulomb branch even when it occurs at strong coupling. The
resulting picture of the stable supersymmetric spectrum is a generalization of
the ``string web'' picture of these states found in string constructions for
certain theories.Comment: 53 pages, 6 figures; more references adde