In the last decade there has been a tremendous increase of interest in studying processes occurring on IS dust. In part this is due to the availability of ground-based and space-borne high quality instruments which have been used to detect molecules in diverse astrophysical environments, from protoplanetary disks to hot cores and dense clouds. It has also been recognized that IS dust has an important role in the formation of molecules, from molecular hydrogen to methanol. Therefore, it is necessary not to study only properties of dust, but also understand how atoms and molecules interact with and on dust.