Knowledge of the atomic scale geometrical structure of matter is a prerequisite for understanding and predicting the properties of technologically and scientifically important materials. The geometrical structure of a material does not only consist of the time and space averaged periodic conformation of atoms in an idealized crystal lattice but also the microstructure caused by imperfections, dislocations, and all kinds of disorder that often are responsible for interesting properties of the material under investigation. The most frequently used technique for the determination of crystal structures is single crystal analysis. However, if no single crystals of suitable size and quality are available, powder diffraction is the nearest alternative. Furthermore, single crystal analysis does not provide information on the bulk material and is not a routinely used technique for the determination of microstructural properties. Neither is it often used to characterize disorder in materials. Studies of macroscopic stresses in components, both residual from processing and in situ under load, are studied by powder diffraction, as is the texture of polycrystalline samples. Powder diffraction remains to this day a crucial tool in the characterization of materials, with increasing importance and breadth of application as instrumentation, methods, data analysis and modeling become more powerful and quantitative.