  I review my work together with Piljin Yi on the spectrum of BPS-saturated
states in N = 2 supersymmetric Yang-Mills theories. In an M-theory description,
such states are realized as certain two-brane configurations. We first show how
the central charge of the N = 2 algebra arises from the two-form central charge
of the eleven-dimensional supersymmetry algebra, and derive the condition for a
two-brane configuration to be BPS-saturated. We then discuss how the topology
of the two-brane determines the type of BPS-multiplet. Finally, we discuss the
phenomenon of marginal stability and show how it is related to the mutual
non-locality of states.
