In his presidential address to the British Association in 1870 James Clerk Maxwell pointed out the desirability of choosing as a reference for defining our units the properties of atoms and molecules, rather than the dimensions, time of rotation, and other properties of our planet.

Since that time there have been many attempts to define a system of units to achieve that end, although the idea of using the properties of atoms has been extended to include using the fundamental constants of physics as appropriate references to define our units. However technical problems have led to many stumbling blocks along the way. The International System of Units, the SI, has developed slowly from its beginnings in the 19th century (although the name "SI" dates only from 1960).

The definitions of the base units have been revised many times as we struggle to achieve Maxwell's desirable goal. This talk will describe a further step forward currently being considered, to redefine four of the base units of the SI, the kilogram, ampere, kelvin and mole, using as references the Planck constant, elementary charge, Boltzmann constant and Avogadro constant respectively. There are still problems to be overcome, but if this change were to be made all the base units of the SI except for the candela would be defined in terms of true invariants of nature, either fundamental constants or atomic properties.

Ian Mills has been a member of the Chemistry Department in the University of Reading since he was first appointed as a Lecturer in 1957. He was made a Personal Professor in Chemical Physics in 1966 and devoted his time to all the usual activities of an academic professor. He pursued research in high resolution molecular spectroscopy of small molecules in the gas phase, in the microwave, infrared and visible/UV regions of the spectrum, until his retirement in 1995. Always having had an interest in the language of science, he became involved with IUPAC in the middle 80s, and was a co-editor of the IUPAC Green Book on 'Quantities, units and symbols in physical chemistry'. From being IUPAC representative on the Consultative Committee for Units at the BIPM, he was appointed President of the CCU in 1995, a position he still holds. He was elected FRS in 1996.