The solidifled inert gases provide a valuable tool for the development of the molecular theory of solids because they approximate to simple theoretical models, in which spherically symmetric molecules are bound in close-packed lattices by short-range, central, van der Wads forces. Experimental investigations have been hampered until recently by the difficulty of producing defect-free single crystal specimens. Now that appropriate techniques have been developed, however, the prospects are good that there will soon be experimental evidence to clarify the extent to which simple two-body potentials may be used to represent accurately the forces in molecular crystals: and to answer related questions concerned, for example, with the contribution of three-body effects. Recent optical experiments suggest that the study of defect and impurity-induced properties in these solids may also prove to be very rewarding, and there are prospects of a condensed inert gas laser. There is considerable theoretical and experimental work still to be completed, however, before these ‘ideal’ solids may be regarded as being fully understood.