A procedure to calculate quantitative rock pressure-temperature (P-T) paths based on chemical zoning profiles in garnet, combined with mineral chemistry from the other minerals in the assemblage, has been applied to samples from diverse tectonic settings with the result that the calculated P-T path is shown to be a sensitive monitor of tectonic processes. Terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the “prograde” P-T path and cooling during decompression in the “retrograde” path. In nappe terranes where hot rocks are emplaced over cool rocks, the upper plate shows cooling during decompression, whereas the lower plate shows heating during compression. In nappe terranes where cool rocks are emplaced over hot, the lower plate shows cooling during compression. Calculation of P-T paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes.