X-ray crystallography is the science of determining the arrangement of atoms within a crystal using X-rays scattered from electrons. The oldest and most exact X-ray crystallographic method is single-crystal X-ray diffraction; for crystals of sufficient purity and regularity, this method gives the crystal structure, which defines the position of most atoms to within a few tenths of an Ångström. Since all atoms have electrons, X-ray crystallography may be used on any type of crystal, be they metals or minerals or engineered semiconductors or crystals of biomolecules such as aspirin or proteins. Such crystal structures have many scientific and medical applications; for example, they can account for unusual electronic or elastic properties of a material, shed light on the molecular events of enzyme catalysis or serve as the basis for structure-based drug design. (wikipedia)