Early in this century, the newly discovered x-ray diffraction by crystals made a complete change in crystallography and in the whole science of the atomic structure of matter, thus giving a new impetus to the development of solid-state physics. Crystallographic methods, pri­ marily x-ray diffraction analysis, penetrated into materials sciences, mol­ ecular physics, and chemistry, and also into many other branches of science. Later, electron and neutron diffraction structure analyses be­ came important since they not only complement x-ray data, but also supply new information on the atomic and the real structure of crystals. Electron microscopy and other modern methods of investigating mat­ ter-optical, electronic paramagnetic, nuclear magnetic, and other res­ onance techniques-yield a large amount of information on the atomic, electronic, and real crystal structures. Crystal physics has also undergone vigorous development. Many re­ markable phenomena have been discovered in crystals and then found various practical applications. Other important factors promoting the development of crystallog­ raphy were the elaboration of the theory of crystal growth (which brought crystallography closer to thermodynamics and physical chem­ istry) and the development of the various methods of growing synthetic crystals dictated by practical needs. Man-made crystals became increas­ ingly important for physical investigations, and they rapidly invaded technology. The production . of synthetic crystals made a tremendous impact on the traditional branches: the mechanical treatment of mate­ rials, precision instrument making, and the jewelry industry.