Rare gas films on graphite exhibit a wide variety of behavior, depending on coverage and temperature. Experimental data for monolayers have been interpreted variously as 2D gases, liquids, and solids. Solid phases are characterized as commensurate, incommensurate or showing orientational epitaxy or domain walls. Theoretical methods have been developed to treat these, beginning with empirical or ab ini tio potentials.Great interest has been drawn to the nature of phase transitions, which show a greater diversity than the analogous 3D phenomena.While some behavior is understood quantitatively, many questions of interpretation remain. G. Benedek (ed.), Surface Properties of Layered Structures, 219-260.