  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.
