This booklet is published on behalf of the Committee for War Prophylaxis of the Netherlands Medical Association. It is written by eleven physicians, half a dozen of whom are English. We do not observe the name of any psycho-analyst among them. We can only welcome the efforts made by the medical profession, especially by medical psychologists, to contribute from their special knowledge something towards our understanding of the baleful phenomenon of war, but it is only too probable that the factors involved are a good deal more complex than is indicated by most of these slight studies.