The direct study of the majority of the physical properties of mantle-forming phases is currently beyond the limits of technology, because of the high pressures and temperatures (greater than 25 GPa and 1800 K) required to simulate lower-mantle conditions. As an alternative to direct study therefore, theoretical and computer-based techniques of lattice simulation and molecular dynamics have been employed to obtain an understanding of the behaviour of high-density silicates. The properties of perfect high-density silicate crystals, including their elastic and spectroscopic characteristics, have been investigated, but to date computer simulations of perfect lattice properties are insufficiently accurate to be used to solve geophysical problems. In contrast, the simulation of phase relations and defects properties are very successful. Small, negative Clapeyron slopes for perovskite-forming transformations are predicted, suggesting that the 670 km discontinuity may not be a rigid barrier to mantle convection. In addition, the activation energies for diffusion in forsterite and perovskite have been calculated, and the suggested high-temperature superionic conductivity of magnesium silicate perovskite has been confirmed.