  In this article we review the conditions for the validity of the
gauge/gravity correspondence in both supersymmetric and non-supersymmetric
string models. We start by reminding what happens in type IIB theory on the
orbifolds C^2/Z_2 and C^3/(Z_2 x Z_2), where this correspondence beautifully
works. In these cases, by performing a complete stringy calculation of the
interaction among D3 branes, it has been shown that the fact that this
correspondence works is a consequence of the open/closed duality and of the
absence of threshold corrections. Then we review the construction of type 0
theories with their orbifolds and orientifolds having spectra free from both
open and closed string tachyons and for such models we study the validity of
the gauge/gravity correspondence, concluding that this is not a peculiarity of
supersymmetric theories, but it may work also for non-supersymmetric models.
Also in these cases, when it works, it is again a consequence of the
open/closed string duality and of vanishing threshold corrections.
