  The discovery of a metallic state and a metal-insulator transition (MIT) in
two-dimensional (2D) electron systems challenges one of the most influential
paradigms of modern mesoscopic physics, namely, that "there is no true metallic
behavior in two dimensions". However, this conclusion was drawn for systems of
noninteracting or weakly interacting carriers, while in all 2D systems
exhibiting the metal-insulator transition, the interaction energy greatly
exceeds all other energy scales. We review the main experimental findings and
show that, although significant progress has been achieved in our understanding
of the MIT in 2D, many open questions remain.
