Every sanitary district is required to appoint a medical officer of health and since 1888 every medical officer of health for a district with a population exceeding 50,000 must have a special diploma in public health. The enforcement of this requirement has done much to raise the standard of work of these officers. It is significant, furthermore, that while in 1873 the percentage of the total population of England and Wales having whole-time medical officers of health was only 20.6, it had increased to 61.4 per cent. in 1911. In the metropolis, in the whole of Scotland, in every English county (forty-four) and in many other districts these officers possess security of tenure, in the sense that they cannot be removed from office without the consent of the Central Government, which usually pays half their salaries. Even without this safeguard, removal from office by the local authority is rare; but there has been long delay in securing the further reform that in all areas the medical officer of health should be able to perform his difficult and sometimes obnoxious duties without fear of removal from office, or of reduction in his emolument, except as the result of deliberate action on appeal to a central authority. When pensions can be earned by medical officers of health and by all medical men on the public health staff, their position will become more attractive for men of good standing; and this reform has become more important in view of the steadily increasing complexity of the medical work now undertaken in a large public health department. It will include _inter alia_ the following officers and activities: superintendent medical officers of health; district medical officers of health; tuberculosis officers; medical officers of maternity and child welfare centres, of venereal disease centres; fever hospitals, and tuberculosis sanatoriums and hospitals. The development of a graduated public health medical service in which each physician employed will be able to develop his own special abilities, will be easier when to the above list is added the work of district (late Poor-Law) medical officers; medical practitioners attending insured persons and such other persons as are treated at the expense of the State; treatment centres for special conditions of the ear, eye, throat; gynecological and other special departments; hospital treatment for general diseases. That there will be development in these directions when the tangle caused by the National Insurance Act of 1911 has been unravelled, there can be no doubt. I have in Lecture IV expressed my opinion as to the additional tangle introduced into the central and local government of the United Kingdom by the National Insurance Act of 1911.