If a beam of ions is directed at a sample surface, then a certain number will be elastically reflected. The intensity of the scattered ions as a function of angle of emission provides information regarding the surface crystallographic structure. The variation in the intensity of the scattered beam is partly due to shadowing of substrate atoms by adsorbed atoms. By use of scattering theory, knowledge the sites that the surface atoms occupy can be derived. This knowledge is complicated by incompletely understood charge neutralisation effects which are important for incident energies below 5keV. Hence for MEIS this is not a problem. The bombarding species in ISS is frequently He+.ISS often refers to low energy ion scattering (LEIS), but can also encompass higher energy forms such as MEIS, HEIS and RBS.