The Source Clays Program of The Clay Minerals Society was initiated in 1972 to distribute a set of reference clays, so that distributed clays could be identical for all recipients. Because most clays do not consist of a single phase, the immediate objective was not to produce a pure product consisting of one clay mineral, but to provide a uniform product. These materials were collected and processed carefully, and sufficient amounts were collected so that material was available for researchers for many years, Large numbers of researchers were thereby assured of working on identical material. Initial descriptions of these materials were presented in the Data Handbook (van Olphen and Fripiat, 1979). An updated version of this book was suggested several years ago because of the availability of new analytical techniques and to provide descriptions of material added to the reference set since 1979.