Incommensurate crystal structures in the elements at high pressure

Recent advances in high-pressure diffraction techniques have revealed remarkably complex crystal structures in the metallic elements at high pressure. In an increasing number of cases, these structures are found to be incommensurate, having either a host-guest composite structure, or modulations of the atomic positions. In this paper we review the structures of these phases, and discuss the insight provided by the structures into the behaviour of the elements at high pressure.