Olivine, the major component of the Earth's upper mantle, is known to contain water in the form of H defects. These defects have a significant effect on the physical and chemical properties of minerals. If we are to correctly interpret seismic data from the upper mantle, and to constrain models of its petrologic and geochemical evolution, then we must have information on the energies and mechanisms of water solubility in olivine, and its effects on physical properties. The aim of this project is to use computer simulation methods to predict the nature of H defects in olivine, their mobility, and their effects on elasticity as a function of pressure, and to use this information to better constrain models of dynamic behaviour of the Earth's upper mantle.