A key principle of quantum gravity postulating that it is possible to obtain information about the bulk of a system, such as a black hole, from the surface of that system. This hypothesis was suggested as a general feature of quantum gravity in the mid-1990s and found to hold in superstring theory a few years later. The holographic hypothesis has turned out to be very useful for performing calculations in quantum chromodynamics and has also been used to analyse various problems in solid-state physics.