I report on a new class of N=1 dualities relating different gauge theories arising from D3 branes probing orientifold singularities. These dualities provide new, nontrivial solutions to the 't Hooft anomaly matching conditions. I discuss an infinite class of orbifold singularities where the duality can be interpreted as S-duality in ten dimensions. I also discuss non-orbifold singularities, and the microscopic interpretation of the duality in terms of an orientifold transition between compact O7+ and O7- planes.
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