For about twenty years, it has been the prevailing view that there can be no
metallic state or metal-insulator transition in two dimensions in zero magnetic
field. In the last several years, however, unusual behavior suggestive of such
a transition has been reported in a variety of dilute two-dimensional electron
and hole systems. The physics behind these observations is presently not
understood. We review and discuss the main experimental findings and suggested
theoretical models.Comment: To be published in Rev. Mod. Phy