Hydrogen under extreme conditions

Some recent advances in our understanding of the properties of hydrogen and deuterium at megabar pressures are reviewed. The emphasis is on recent spectroscopic experiments to elucidate the nature of the H-A phase (Phase III), the high-pressure phase diagram, and on the reported generation of liquid metallic hydrogen under shock compression conditions. Density Functional and Quantum-Monte Carlo calculations have proved a useful guide to interpreting the experimental results: these will also be reviewed.