Neutrons provide a unique tool for the understanding of both surface and deep Earth processes. In the former case, which is dominated by the effects of water, the sensitivity of neutrons to hydrogen allows precise measurements to be made of the structure, synthesis and dynamics of hydrous mineral phases. In the latter case, which utilises the penetrating power of neutrons to study large volume samples coupled with the sensitivity of neutrons to light atoms, the field can be subdivided into either petrophysics, the understanding of the properties of rocks at high temperatures and pressures, or mineral physics, the detailed study of the individual constituent mineral phases. ISIS has been at the forefront in the application of neutrons to both fields with the first ever rock deformation experiments and the first in-situ studies of the ordering of cations into rock-forming minerals being made on Polaris. These early experiments have been extended into detailed studies of the texture and rheological behaviour of complex multicomponent rocks using Gem and Engin-x and studies of the changes in crystal structure of rock-forming silicates with temperature using Hrpd and Polaris, and in pressure, using Pearl.