The structure and function of a new class of host molecules for the supramolecular complexation of anionic guest species are analysed within the context of other recent advances in the field. In particular, organometallic hosts basd upon the calixarenes, and the related macrocycle cyclotriveratrylene (CTV), are examined. X-Ray crystallographic results clearly demonstrate the inclusion of anionic guest species such as BF₄^–, I^–, CF₃SO₃^–, ReO₄^–etc. within the ostensibly electron-rich bowl-shaped cavities of both types of host as a result of cooperative effects arising from the presence of two or more metal centres arranged around a common binding pocket. Solution radiochemical studies show that hosts based upon CTV in particular are selective for large tetrahedral anions such as MO₄^–(M = Tc, Re). It is anticipated that the ability to discriminate between anions on a size and shape selective basis by means of manipulation of host cavity dimensions will pave the way towards new sensor devices and methods of environmental waste remediation.