To solve a crystal structure means to determine the precise spatial arrangements of all of the atoms in a chemical compound in the crystalline state. This knowledge gives a chemist access to a large range of information, induding connectivity, conformation, and accurate bond lengths and angles. In addition, it implies the stoichiometry, the density, the symmetry and the three dimensional packing of the atoms in the solid. Since interatomic distances are in the region of 100-300 pm or 1-3 A, I microscopy using visible light ( wavelength Je ca. 300-700 nm) is not applicable (Fig. l. l). In 1912, Max von Laue showed that crystals are based on a three dimensional lattice which scatters radiation with a wavelength in the vicinity of interatomic distances, i. e. X -rays with Je = 50-300 pm. The process by wh ich this radiation, without changing its wave length, is converted through interference by the lattice to a vast number of observable "reflections" with characteristic directions in space is called X-ray diffraction. The method by which the directions and the intensities of these reflections are measured, and the ordering of the atoms in the crystal deduced from them, is called X -ray struc ture analysis. The following chapter deals with the lattice properties of crystals, the starting point for the explanation of these interference phenomena. Interatomic distances Crystals . . . . . . . . . .