In Paris I shared the alternating hope and grief of the inhabitants during the course of that great battle, and had the constant worry of foreseeing a long separation from my children in case the Germans succeeded in occupying the city. Yet I felt that I must stay at my post. After the successful outcome of the battle, however, any immediate danger of occupation being removed, I was able to have my daughters come back from Brittany to Paris and again take up their studies. This was the great desire of my children, who did not want to stay away from me and from their work, even if many other families thought it wiser to stay in the country, far from the front. The dominant duty imposed on every one at that time was to help the country in whatever way possible during the extreme crisis that it faced. No general instructions to this were given to the members of the University. It was left to each to take his own initiative and means of action. I therefore sought to discover the most efficient way to do useful work, turning my scientific knowledge to most profit. During the rapid succession of events in August, 1914, it was clearly proved that the preparation for defense was insufficient. Public feeling was especially aroused by the realization of the grave failings which appeared in the organization of the Health Service. My own attention was particularly drawn to this situation, and I soon found a field of activity which, once entered upon, absorbed the greatest part of my time and efforts until the end of the war, and even for some time thereafter. The work was the organization of radiologic and radiotherapeutic services for the military hospitals. But I also had to make the change, during these difficult war years, of my laboratory into the new building of the Institute of Radium and to continue, in the measure possible to me, regular teaching, as well as to investigate certain problems especially interesting the military service. It is well known that the X-rays offer surgeons and doctors extremely useful means for the examination of the sick and wounded. They make possible the discovery and the exact location of projectiles which have entered the body, and this is a great help in their extraction. These rays also reveal lesions of bones and of the internal organs and permit one to follow the progress of recovery from internal injuries. The use of the X-rays during the war saved the lives of many wounded men; it also saved many from long suffering and lasting infirmity. To all the wounded it gave a greater chance of recovery.