Ultrahigh pressures and temperatures can now be generated statically in the laboratory using high power optical lasers, which allow for measurements of the equation of the state, shock-induced phase transitions, melting models and high-pressure polymorphism, so on and so forth, of materials. Lattice-level in situ X-ray diffraction (XRD) measurements are vital to probe these mechanisms under dynamic compression and decompression. Traditionally, two different methodologies for investigating a material’s atomic arrangement in the solid state are imaging, which provides real space information, and x-ray scattering, which provides reciprocal space information. Imaging techniques such as scanning electron microscopy, and transmission electron microscopy focus on a small area at atomic or near atomic dimensions, while XRD techniques sample a much larger volume of material. R&D on advanced XRD techniques including Rietveld refinement, structure solution, crystallite size determination, percent crystallinity, residual stress, and orientation (texture analysis) are highly useful for a precise measure of the crystal microstructure, phase identifications. This special issue targets on research of the state-of-art X-ray diffraction techniques dedicated for physics under high pressure and high temperature conditions.