  I explain two applications of the relationship between four dimensional N=1
supersymmetric gauge theories, zero dimensional gauged matrix models, and
geometric transitions in string theory. The first is related to the spectrum of
BPS domain walls or BPS branes. It is shown that one can smoothly interpolate
between a D-brane state, whose weak coupling tension scales as Nc or 1/gs, and
a closed string solitonic state, whose weak coupling tension scales as Nc^2 or
1/gs^2. This is part of a larger theory of N=1 quantum parameter spaces. The
second is a new purely geometric approach to sum exactly over planar diagrams
in zero dimension. It is an example of open/closed string duality.
