Among new fields of application of mathematics that have arisen since the war one of the more important is the theory of nuclear reactors. The reader need hardly be reminded of the role that atomic energy is bound to play in our future. As an indication, however, of the amount of unclassified research that has to go and is going into the development of its peaceful applications it can be mentioned that there are now several scientific periodicals both in the U.S.A. and in Britain entirely devoted to problems appertaining to nuclear reactors. Much of this work, in particular most of the work regarding the diffusion and slowing down of neutrons, is entirely mathematical in nature.