The arrival at our present stage of understanding of atomic and nuclear structure has been greatly complicated by the fact that one's thought pattern has had to be modified with respect to everyday experience in order to gain a truer understanding of atomic phenomena. An example of this, useful for the present purpose, is the dual characterization which is associated with atomic or nuclear particles in their behavior as particles or as waves. Neutrons are of this nature, and although they are invariably referred to as particles, we know them to have the characteristics of both waves and particles. This fact makes it difficult for one to form a sensible mental picture of a neutron and somewhat in desperation we have been led to visualize it as illustrated in the accompanying caricature. This picture may be quite different from the one that you have in mind or that which might be drawn by someone else, but its chief merit is that it possesses features which describe most of the properties which we know to be associated with neutrons. Indifferent to the charge of a nucleus or its electrons, neutrons can penetrate where charged particles can not. Their use to probe the structure of matter is opening up the promising new field of neutron diffraction.