Open Strings and Dualities

In the fruitful interplay between gauge fields and strings and in many conjectured M-theory dualities, open strings play a prominent role. We review the construction of open-string descendants (un-orientifolds) of closed-string theories admitting a generalized orientation reversal involution. We then specialize the construction to some classes of non-supersymmetric models in D = 10 that have been recently considered in the context of duality without supersymmetry. We also discuss the propagation of open and unoriented strings on the NS pentabrane (N5-brane). This background is a prototype of the configurations of branes and orientifold planes that represent a powerful alternative to the geometric engineering of Supersymmetric Yang-Mills Theories. The resulting description of D-branes in non-trivial backgrounds looks very different from the one naively expected. In particular the very distinction between different Dp-branes becomes ambiguous in the presence of strong curvature effects.