The positions of the atoms in a material define its atomic structure. In a gas or liquid the atoms move throughout the available volume, and the structure is described using one or more pair distribution functions which are probability distributions of instantaneous distances between pairs of atoms. The atoms in a solid behave quite differently, for the most part moving back and forth abouttheir equilibrium positions, and pair distribution functions can again be used to describe the structure. The pair distribution approach is most useful when applied to non-crystalline solids such as glasses, in which the mean (time averaged) positions of the atoms lack long range order. In crystalline solids the mean positions of the atoms have a regularity that persists over large distances in all directions, and the structure is specified by the mean positions of the atoms (together with information that describes site occupancies and mean displacement amplitudes).