Although educational and developmental psychology should inform each other, it is unusual when they do so. Three reasons are offered for the differences between the two psychologies. First, the content of research and purposes for choosing that content are different. Second, the significance of developmental sequences is different for the two psychologies, and the role of values in developmental sequences also differs. And third, there are differences in the search for biologically constrained laws of human cognitive development versus schooling effects. It is suggested that the two psychologies can inform each other. The zone where the two psychologies meet, the zone of educational-developmental psychology, is an emergent area which is neither developmental nor educational psychology as traditionally practiced, but it takes elements from both.