We give an overview over the subject of gauge/gravity duality and its applications. Based on string theory, gauge/gravity duality maps strongly coupled quantum field theories to weakly coupled gravity theories and thus provides a new framework for studying physical phenomena in strongly coupled systems. We describe the main features of this approach and discuss its range of applicability. Moreover, we give examples of applications in both elementary particle/nuclear physics and condensed matter physics. The examples presented include the quark-gluon plasma, mesons, quantum phase transitions and superconductors.