The oldest of the professions and the youngest of the sciences have become interdependent without knowing it. The dependence of medicine upon the physical and biological sciences has long been recognized. Early in the twentieth century psychology and medicine began to interpenetrate more than ever before. The social elements in the development of personality have been explored during the same period by sociologists as well as psychologists and are being incorporated somewhat haltingly into the studies and thinking of psychiatrists. From the point of view of social function, the application of medical knowledge has become increasingly dependent upon group action. The individual physician and his patient can no longer constitute the universe within which medical knowledge is developed and applied. Conversely, the recent development and the present functioning of medicine cannot be understood without studies in the social as well as the medical sciences. These statements require illustration. The attempt to control communicable disease through group action is an example of social factors in the application of medical knowledge which runs back far into human history. During the past fifty years the knowledge and techniques of surgery have been made capable of safe and widespread application through the transformation and expansion of an ancient social institution-the hospital. There the science and technology of medicine can be seen in daily interplay with social and economic elements. The discovery early in this century of means for detecting susceptibility to diphtheria and of immunizing susceptible children against it has been capable of large application only through organized social action, in which public-health departments and schools have been the chief agents, along with such social institutions as the family and the organized medical profession. Professor Ogburn says, "That marvelous grouping which we call a city is a result of the transportation inventions and stationary machines using non-animal sources of power."' He might have added, "and of the sanitary inventions, also." For the modern city would have a death-rate some three times its present level and would be swept periodically by devastating epidemics without those inventions which have made possible the organized control of water, milk, and food supplies. Recent advances in