Aspects of gauge/string dualities

In this thesis we investigate on the role that dualities play nowadays in our under-standing of string and Quantum Field Theories.The rst part is devoted to holographic dualities, where the case of the string dual of certain one-, two- and four dimensional eld theories is explored in detail. We begin in Chapter 1 by discussing the holographic dual of a broad class of four-dimensional eld theories. We specialise to the case of quiver eld theories where a number of formulas computing charges, number of branes and Linking Numbers are given. We then introduce marginal deformations in the supergravity backgrounds to uncover innite new families of solutions to type II supergravity and M-theory. In Chapter 2, we discuss spin 2 uctuations around a class of warped AdS3 backgrounds. We identify explicitly the dual operators of a given protected sector. Also, a formula for the central charge of the dual two-dimensional eld theories is derived. In Chapter 3, we introduce two new classes of geometries with an AdS2 factor. We outline the importance to black hole physics and in one case give a prescription for the dual eld theories.The second part is devoted to QFT/QFT dualities in Chern-Simons theories, and how dualities between Chern-Simons theories in three dimensions can be motivated in string theory. It is found that a known duality between three-dimensional Chern-Simons theories with unitary gauge symmetry can be motivated by a brane setup in Type 0B string theory with an orientifold. In particular, the phase diagram of unitary QCD3 is inferred from a dual theory.