Recent developments in Raman spectroscopy at high pressure, high temperature and combined high pressure and variable temperature are presented. The instrumental and technical aspects of Raman spectroscopy, and coupling of diamond anvil cells and miniature furnaces to Raman microspectrometers are discussed. Several pitfalls are dis cussed: (1) the generation of shear stresses on the sample during room temperature compression and their effects on phase transitions; (2) thermal radiation during high temperature measurements: (3) thermal pressure in laser heated diamond anvil cells. The application of in-situ Raman spectroscopy to the study of phase transitions occurring at extreme pressures is illustrated by the new recently discovered phase transformations in the SiO2 system: quartz I --> quartz II or pressure-induced amorphization of quartz. Finally, it is shown how vibrational mode anharmonicity can be obtained from the pressure-and temperature-induced shifts of Raman modes. Accurate determination of these shifts in conjunction with well-defined equation of states permits a separation of the temperature dependence of a phonon frequency into its two components: the implicit volume-driven (quasi-harmonic) contribution produced by thermal expansion and the explicit phonon-excitation (intrinsic anharmonicity) contribution or phonon-phonon interaction. This last point is illustrated by the example of the hard Raman modes of Mg2SiO4-forsterite and SiO2-quartz.