The gauge/string theory duality is one of the most revolutionary ideas in the realm of modern theoretical physics. A concrete realization of such a duality has been provided by the AdS/CFT conjecture, stating a holographic correspondence between string theory on a given curved background and a four-dimensional superconformal field theory. This has led to crucial conceptual advances in two longstanding challenges: the understanding of the strong coupling dynamics of gauge theories and that of the microscopic nature of black holes. As it stands, the original correspondence applies to a limited class of situations. Many steps have been taken towards extending it, in order to add flavour degrees of freedom to the gauge theory side and to break part of its symmetries. A restriction also stems from the fact that it is tractable only in certain regimes, corresponding to weakly curved gravity backgrounds. The general framework of this project will be to consider a particular setting where the duality can be probed beyond the usual approximations. This can be done by using exact string backgrounds whose worldsheet conformal field theory description is known. Recently, very impressive checks of the AdS/CFT correspondence have been performed in this context beyond the usual supergravity approximation. We would like to use similar tools to study extensions of the duality in a completely controlled setting. We will first consider adding flavour degrees of freedom into the dual theory. This will be achieved by adding appropriate D-branes in the exact string background. Knowledge of the boundary state description of these D-branes would then allow us to take their back-reaction into account. Next, we will study the holographic dual theories of a class of exact backgrounds obtained as deformations of the original one. This will allow us to obtain a more general class of gauge theories, as well as a microscopic description of a variety of new black objects.