Observations of the phenomenon of ion exchange date from ancient times. The ‘mechanism’ of the reaction, however, was established in 1850 by two English chemists, H. S. Thompson and J. T. Way, but it is only in the past few decades that the subject has expanded to become a true science from which extensive industrial applications have emerged. Progress in both the theoretical understanding and process developments of ion exchange has been rapid with the one complementing the other. In the academic sphere, the growth of knowledge has paralleled closely the physical chemistry interpretation of the behaviour of concentrated solutions of electrolytes. This is not unexpected since ion exchange materials are in effect electrolytes, albeit mostly solid ones, and in some respects they behave in an identical fashion.