  We propose a general method to study open/closed string dualities from
transitions in M theory which is valid for a large class of geometrical
configurations. By T-duality we can transform geometrically engineered
configurations into N = 1 brane configurations and study the transitions of the
corresponding branes by lifting the configurations to M-theory. We describe the
transformed degenerated M5 branes and extract the field theory information on
gluino condensation by factorization of the Seiberg-Witten curve. We also
include massive flavors and orientifolds and discuss Seiberg duality which
appears in this case as a birational flop. After the transition, the Seiberg
duality becomes an abelian electric-magnetic duality.
