With other practitioners — with physicians and lawyers, for ex ample — there was always a basis of knowledge held in common. If a neurologist had occasion to confer with a surgeon, each could assume in the other a mastery of the elements of a whole group of basic sciences and of the formulated and transmitted experience of his own guild besides. But what common knowledge could social workers assume in like case? This was my query of fifteen years ago. It seemed to me then, and it is still my opinion, that the elements of social diagnosis, if formulated, should constitute a part of the ground which all social case workers could occupy in common, and that it should become possible in time to take for granted, in every social practitioner, a knowledge and mastery of those elements, and of the modifications in them which each decade of practice would surely bring.