Colloid science has been applied by soil chemists and clay mineral ogists for many years, and some of the most important studies on the behavior of colloids have been contributed by them. Barring a few notable exceptions, only in the last decade have geochemists applied colloid science in their research and in this period much work has been published. It seemed to the authors that it would be useful at this stage to attempt to summarize the progress made and to try to examine what colloid science has contributed and can further contribute to geo chemistry. This book is based partly on a course of the same title given to graduate students by one of the authors (S. Y) between 1972 and 1977 at the Department of Geology at the Hebrew University of Jerusalem. Consequently many fundamental concepts of the subject are included that will be of use to graduate students in geology, geo chemistry, soil science, and oceanography. So that specialists interested in certain sections may find their subjects comprehensively covered, a few topics are dealt with in more than one chapter so that readers may ignore sections not especially of interest to them. However the chapters more fully treating certain topics are cross-referenced. In such cases the subjects are treated from different viewpoints and the citations used represent these dif fering viewpoints.