A prerequisite for understanding the physico-chemical properties of materials under high-pressure and high-temperature conditions is knowledge about their atomic-scale structures. Such information on the structure-property relationships is vital for being able to predict a material’s response to extreme environments, which impacts on fields ranging from geophysics to materials processing and nuclear fusion. Diffraction is one of the premier techniques for solving the structure of crystalline and amorphous materials, however, it has not been applied extensively at high-pressures and high-temperatures because of the limitations associated with existing instrumentation. Working in collaboration with the high pressure teams at the ISIS Neutron and Muon Source and the Diamond Light Source, we aim to solve this problem by developing an easy-to-use internal heating system for the Paris-Edinburgh press that will enable the structure of materials to be measured at pressures up to 20 GPa and temperatures up to 2000 K with minimal background scattering. This range of extreme conditions will allow, inter alia, access to the conditions found in the Earth’s upper mantle, in which atomic-scale structure plays a key role in determining the connectivity and hence the mechanical and rheological properties of silicate network structures.