Present-day society urgently needs reliable measures, both local and national, of the health of the people. Declines in death rates undoubtedly indicate great improvements in public health in terms of survival, but do not necessarily represent equal declines in the numbers of the sick and the magnitude of their needs. More precise knowledge is needed of ailments and diseases in specific communities, as a factual basis to administer present services for the sick, to interpret local trends of morbidity, to enlarge the content of epidemiology and translate its findings into preventive measures, and to plan new services in changing conditions or newly developing areas. But this knowledge must be specified in precise terms. "Total" knowledge of a community is an abstract concept, in reality unattainable. Medicine does not deal with the total person, nor does public health administer the total community, but each selects certain features which to the doctor or administrator seem important. In social medicine we must consider it important to seek knowledge concerning the preventable ill-health of communities in order that preventive action may be taken. Since prevention and the measurement of the preventable are aspects of a single process, and since the normal agencies of prevention are local, the laboratory of social medicine is the local community, and its interest is focussed on the social group rather than on individuals. The life and circumstances of the small community or "neighbourhood" are dominant influences on the health of the individual in an urban society. This small definable community we take to be the unit of research: and we judge as important those environmental factors which can foster or impair the life and well-being of the social group. We therefore select for investigation those aspects of social and health experience which relate particularly to the pro motion of the health of the group in its own local environment.