Theoretical approaches to the metal-insulator transition in 2D

Recent experiments on the two-dimensional electron gas in various semicondu ctor structures have revealed the existence of what appears to be a continu ous low-temperature metal-insulator transition as the electron density is v aried, although it was previously commonly accepted that all electronic sta tes will be localized in a two-dimensional disordered medium. Theoretical p roposals for the explanation of various aspects of the experiments will be reviewed and implications for the scaling theory of localization will be di scussed.