  By embedding N=2 gauge theories in string theory and utilizing string
dualities we map the counting of BPS states with arbitrary electric and
magnetic charges to computations of an A-model topological string on an
associated geometry constructed from the data of the SW curve. We show how the
conjecture of Kontsevich and Soibelman regarding wall crossing, as well as a
more refined version which captures the spin content of BPS states, is a
natural consequence. Chern-Simons theory realized on A-branes and a twistorial
construction play key roles.
