This chapter describes how X-ray structural measurements can be done on molten sili-cates under high pressures, 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.