In the title of this essay the traditional partner of development—growth—is missing. We have been taught that growth refers to the multiplication of cells and size changes, while development concerns the maturation of such structures and their concomitant functions. Today we realize that the whole process—call it what you will—is extremely complex and a mass of processes interacting upon each other. Because, in addition, there is no dividing line between the two terms, it seems desirable to drop an unnecessary and rather false division and use but one of the two good words. PART I. A GUIDE TO THE INTERPRETATION OF GROWTH-CHARTS AND DEVELOPMENT ASSESSMENTS The pediatrician is confronted today with many aspects of development. Are babies who are fed two different regimens growing comparably? Is a patient receiving steroids being stunted? Or does an anabolic hormone hasten epiphysial closure to the extent that it causes premature arrest of growth? Before interest can be taken in such questions, and in the problems of identifying true disturbances in developmental patterns, the basic principles of human development, of present methods, and the resources available must be critically reviewed. The pediatrician and his ancillary colleagues may well be overwhelmed by the quantity of data, charts and grids concerning growth that have appeared in the last 20 years. This is not surprising and not the fault of physicians. It is perhaps an irritating fact that development and its allied problems cannot be discussed without a basic, simple knowledge of biometiy. The reader can be assured that the knowledge he requires is not complicated and will stand him in good stead in many branches of his scientific art.