The study of solids, at static pressures up to 500 kilobars, and transient pressures up to ∼ 5 × 106bars (1 bar = 0.99 atmospheres), has increased rapidly in the last ten years. Under pressure, all solids show a variety of atomic and electronic transformations into more condensed forms, eventually approaching a metallic state. As well as providing novel forms of matter to study, these experiments give data on the stability and cohesion in different types of crystal, the transition from the insulator to the metallic state, equations of state, melting phenomena and geophysical problems. In this review the experimental methods are briefly surveyed, and some of the more important results on typical ionic, covalent, metallic and molecular crystals are discussed.