This is a very timely book which should become essential reading for psychologists and people working in the areas of primary education and secondary mathematics. This collection of papers goes a long way to make research on the development of mathematical knowledge accessible to teachers while in no way compromising its scholarship. teachers and psychologists interested in mathematical development will do well to invest time and effort in reading and re-reading this collection in order to extend their understanding of the processes involved in both learning and teaching. - Review in British Journal of Developmental Psychology, 17:4, 1999, by Rhona Stainthorp, University of Reading.