The structure of physisorbed rare-gas layers depends on the interplay between the mutual interaction of the rare-gas atoms and the periodic corrugation of the substrate holding potential. A fascinating variety of structural phases and of mutual phase transformations of these layers adsorbed on graphite and on Pt(111) has been observed. This variety is due to the delicate balance between these interactions. This balance is also the cause for radical changes of salient layer features induced by minute amounts of impurities and defects. High-resolution thermal He scattering appears to be particularly appropriate for an exhaustive investigation of the properties of these layers: simple diffraction patterns (no substrate scattering contribution) including sensitivity for shallow periodic buckling as well as high-resolution (<0.4 meV) energy-loss spectra are the main capabilities which allow for a complete analysis of the structure and dynamics of these layers. The extreme sensitivity for surface defects and impurities, and the absence of any disturbance induced by the He beam are further advantages.