The appearance of this special issue marks a crescendo in the interest in trace-element partitioning studies led by the application of several new microbeam analytical tools for the in situ measurement of trace elements. Results from these microbeam methods have also led to restored confidence that experiments conducted with artificially increased levels of trace elements and analysed by electron microprobe (EMP) can give meaningful partition coefficients which can be applied to natural systems. As a consequence, there has been a recent substantial increase in the number of studies, manifested in symposia in London, U.K. (organised by Richard Hinton) and Baltimore, Maryland, U.S.A. (organised by James Brenan and Tom Skulski) in 1993 and by a special program on element partitioning (coordinated by Al Hofmann) run by the Deutsche Forschungsgemeinschaft in Germany. Contributions to this special issue come from these three sources, and during their time "in press" there will have been another international symposium on partition coefficients at the Goldschmidt Conference in Edinburgh, U.K., in August-September 1994. The much-cited proceedings of the 1977 Sedona conference, published as a special issue of Geochimica et Cosmochimica Acta in 1978, thus receive a long overdue update.