High-pressure and high-temperature in situ X-ray diffraction in a multianvil apparatus is the most reliable method for determination of phase boundaries of high-pressure transitions. Several basic experimental techniques are reviewed in this article. Recommendations include the equations of state for NaCl, MgO, and Au given by Brown (1999), Matsui et al. (2000), and Fei et al. (2004a), respectively, for practical use, as well as phase boundaries given by the following studies: coesite-stishovite transition in SiO₂ by Zhang et al. (1995), olivine-ringwoodite transition in Fe₂SiO₄ by Yagi et al. (1997), and olivine-wadsleyite...