WHILE scientific men of all kinds have been called upon to use their skill and knowledge for the nation's service in the present War, certain classes have been in especial demand, and the physicist is ranked among these 'scarcity categories'. The demand for physicists exceeds the supply, and universities have been pressed to train as many men as possible to fill the gap. There was no such general demand for the services of scientists in the War of 1914-18. Although there were some technical applications which called for trained scientific men, in the main that war was fought with comparatively simple gear. Physicists are now being used to develop and use all the lighter apparatus of war, the instruments for communication, for detection of aircraft and submarines, and for direction of our batteries. They are increasingly applying their analytical technique to operational research into the most effective way of using our weapons. Many of these latest developments are directed by men supplied by our industrial or university laboratories.