The effect of pressure (up to 22 GPa) and temperature (to 1300 K) on the Raman spectra has been measured for pyrope, grossular and andradite garnets. No phase changes were observed and all Raman modes increase or decrease linearly with pressure and temperature, respectively. Generally the high-frequency modes (800-1000 cm-1) have larger shifts than those lying below 650 cm-1. Anharmonic parameters a(i) almost-equal-to (partial derivative lnv(i)/partial derivative T)v, calculated from the measured pressure and temperature shifts of all modes, are negative, with lower absolute values for the stretching vibrational modes of the SiO4 tetrahedra (a(i) almost-equal-to 1.5x10(-5) K-1) than for the lattice modes (a(i) almost-equal-to 2.5x10(-5) K-1). For garnets and olivines correlations between the mean frequency shift MFS (MFS=SIGMA(partial derivative lnv(i)/partial derivative P)/n) and crystal bulk modulus (K(T)) are found for the bending modes of SiO4 polyhedra and lattice modes. A similar trend is observed between the MFS of all modes and K(T) for most orthosilicates for which spectroscopic data at elevated pressures are available. This trend is accounted for by the compression mechanism in orthosilicates which involves only minor polyhedral tilting and important bond length shortening. Minerals for which the dominant compression mechanism is polyhedral tilting do not follow the trend of orthosilicates (e.g. quartz and perovskite.) A similar trend is observed between (SIGMA(partial derivative lnv(i)/partial derivative T)/n) and the coefficient of thermal expansion.