Since the great discovery made by Laue, x-ray diffraction has become the most im­ portant method for the investigation of atomic structure in condensed matter. Cer­ tain investigations, however, are difficult or totally impossible to conduct using x-rays, for example, the localization of atomic nuclei or atoms having only a few core electrons, and the observation of magnetic moments. The investigation of these important areas is made possible by neutron diffraction. Thus this method has devel­ oped into an important supplement to x-ray investigations. An ever-increasing de­ mand is put on the method by research areas, old and new. Neutron diffraction has completely reformed the subject of solid-state magnetism; especially the area of chemical binding has received a new impetus by the union of x-ray and neutron dif­ fraction. An exchange of different isotopes of the same element gives rise, as a rule, to a change in the strength of the neutron diffraction. Due to this effect itls possible, by means of deuteration, to make visible a single chain in a solid high polymer. Thus neutron small-angle scattering is important in protein research and for the biology of macromolecules. Of equal importance is the application of neu:ron diffraction in metallurgy. There already exist several excellent books which discuss the results obtained by neans of neutron diffraction.