text: The origins of the field date back to the 1950s, when Yehuda Hirshberg developed the photochromic spiropyrans and suggested their use in data storage. In the 1970s, Valerii Barachevskii demonstrated that this photochromism could be produced by two-photon excitation, and at the end of the 1980s Peter M. Rentzepis showed that this could lead to three-dimensional data storage. A wide range of physical phenomena for data reading and recording have been investigated, large numbers of chemical systems for the medium have been developed and evaluated, and extensive work has been carried out in solving the problems associated with the optical systems required for the reading and recording of data. Currently, several groups remain working on solutions with various levels of development and interest in commercialization.