X-ray diffraction was first utilized in establishing the atomic structure of crystals. Later the technique of X-ray diffraction found other applications, however, and branched off from pure crystallography, extending to studies of imperfections in crystals, sizes of crystallites, and even to studies of the atomic structure of amorphous bodies. These fields of application of X-rays were made possible by further developments in the theory of the diffraction of X-rays by matter and also by improvements in experimental methods.