Though for a number of years prior to the war there had been a steadily increasing appreciation of the importance of neurology and psychiatry in the organization of a fighting-machine, the theories which had been evolved along these lines were never put into practice, at least on a large scale, until America’s entry into the great conflict, when there was organized the Neuropsychiatric Section of the Division of Internal Medicine. When the section was created, about fifty neuropsychiatric officers were commissioned; when the Armistice was signed, this number had risen to nearly 700. The chief function of the section was the exclusion from the army, by means of special tests, of men who, because of mental and nervous diseases, were considered unfit for military service. At first this section was treated with open derision or contemptuous tolerance by certain of the narrow-minded or the prejudiced—for the Medical Corps, like all other branches of the army, is not without its fogies who regard with suspicion anything that is new. The best proof of the success of its work, however, is the fact that it discovered the presence in the army, at home and overseas, of more than 72,000 men suffering from nervous and mental disorders, every one of whom was a potential menace to our success as long as he remained in active service. Thanks to the simple but highly effective tests which the psychiatrists devised, certain men were discovered to be moral perverts; the tests showed that others, if exposed to the strain of battle, probably would have suffered mental collapse, and that still others did not possess a sufficiently developed mentality to understand or to carry out orders. Imagine how grave a menace a single pervert might have proved to the morals of the men with whom he was associated in the intimacy of army life. Picture the danger to the success of a military operation of a single soldier who did not possess sufficient intelligence to understand the orders which were given him or the courage to carry them out. Such men were of far greater potential danger to the welfare of the army than were those suffering from tuberculosis. By means of the psychiatric tests given at the camps and cantonments, more than 1 per cent of all the men brought into the army by the draft were discovered to be mentally unfit and were at once rejected. On the other hand, many drafted men were found to possess exceptional mental qualifications and were thus marked out for assignments where their special aptitudes would prove of the greatest value, in many cases being recommended for the officers’ training-camps. This was the first war in which mental tests have been employed. Men with undeveloped minds, unstable nervous systems, or inadequate self-control are very bad risks for armies. They are unknown quantities and their behavior in moments of stress cannot be relied upon. Such men may cause disaster in action, they are liable to shell-shock, and they are likely