  The interplay between strong Coulomb interactions and randomness has been a
long-standing problem in condensed matter physics. According to the scaling
theory of localization, in two-dimensional systems of noninteracting or weakly
interacting electrons, the ever-present randomness causes the resistance to
rise as the temperature is decreased, leading to an insulating ground state.
However, new evidence has emerged within the past decade indicating a
transition from insulating to metallic phase in two-dimensional systems of
strongly interacting electrons. We review earlier experiments that demonstrate
the unexpected presence of a metallic phase in two dimensions, and present an
overview of recent experiments with emphasis on the anomalous magnetic
properties that have been observed in the vicinity of the transition.
