Implementation and regulation of the molecular mechanisms underlying biological processes is dependent on direct interactions between biological molecules. These interactions are characterised by specific binding between at least one molecule and another, and for binding to occur the molecules must be able to come close enough to each other to make contact. One of the partners in the interaction is invariably a macromolecule (e.g. protein or DNA) or an assembly of large size, such as a lipid bilayer. The interaction may take place with the partners in solution, or with at least one attached to a biological surface (e.g. a membrane) or a very large structure such as a chromosome. Where the partners are part of a membrane or other large structure then there must be a mechanism, such as lateral diffusion in the plane of the membrane, that permits them to come together close enough for interaction.