IN the period between the War of 1914–18 and the beginning of the present War, crystallography experienced a rejuvenation, and in its new form of X-ray crystallography it has once more attained a central position bordering on many fields of promising development. While in its early days crystallography had contacts mainly with mineralogy, geology and crystal optics, at present its most intimate links are with atomic physics, chemistry, metallography and increasingly with engineering and industrial production. In fact, crystallography is forming the background for most problems concerning the solid state of matter. Compared to its advances on the newly won ground, its recent interactions with mineralogy and geology have been rather limited, but this may change in the future. It is recognized that crystallography stands in respect to its methods nearest to physics, for it originated as a part of physics; but the most urgent present demand for its methods and data comes from chemistry.