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 transition in Mg₂SiO₄ by Morishima et al. (1994). Although the akimotoite-perovskite transition in MgSiO₃ and dissociation of ring-woodite to perovskite + periclase in Mg₂SiO₄ have been studied repeatedly, no reasonable agreements have been established to date. Although the wadsleyite-ringwoodite transition in Mg₂SiO₄ and dissociation of garnet to perovskite + corundum have also been studied, the results are still preliminary. The present state-of-the-art experimental techniques have several problems in accurately determining positions of phase boundaries, namely: (1) uncertainty of temperature distribution in a high-pressure cell, (2) an unknown pressure effect on electromotive force (EMF), (3) low equation of state reliability for pressure determination at relatively low pressures and high temperatures, (4) pressure intensification and reduction of pressure standards that are mixed with other materials to prevent grain growth, and (5) sluggish kinetics.