The recent application of the scattering of thermal He beams to the study of disordered adsorbates is featured by both an extreme sensitivity (coverages down to θ≈ 0.001 are accessible) and non-destructiveness. The sensitivity is a consequence of the very large cross-section (ca. 100 Å) for diffuse scattering exhibited by adsorbates. The cross-section overlap is substantial even at low coverages and the degree of overlap depends on the nature of the adsorbate distribution along the surface, i.e. on the mutual interaction of the adsorbates. Thus information on the latter can be obtained from the former; this is the overlap approach. A recent development has shown that this is also true if ‘monovacancy’ is substituted for ‘adsorbate’. Examples of the use of the overlap approach are given. Information, inaccessible so far, on the mutual interaction of adsorbates and of vacancies, on the details of the adsorption and of the vacancy-creation processes and on two-dimensional phase transitions are obtained. Similarities and differences between adsorbate and vacancy behaviour and their influence on thermal He scattering are discussed. Finally, a mixed overlap between adsorbates and defects is used to investigate adsorbate diffusion.