This chapter describes how the structure of molten silicates under high pressures may be measured by synchrotron X-ray diffraction, using either large-volume presses or diamond-anvil cells, the latter combined with resistive-heating or laser-heating techniques. A brief summary of the data obtained so far is given, followed by a description of both energy-dispersive and angle-dispersive techniques, including challenges and how they may be overcome. Three areas of research are then highlighted: (1) structural measurements at extreme pressure conditions up to 100 GPa, (2) tracking the structural environment of minor/trace elements in magmas, and (3) the different ways to obtain the density of melts from X-ray diffraction data. Finally, some future prospects are discussed
