Zircon is of fundamental importance in the investigation of deeply subducted crustal rocks in which it is a trace constituent. Tiny mineral inclusions within zircon may be the only indicators that rocks were subducted to a depth of up to 150 km. Because zircon is resistant to physical and chemical changes, it preserves stages of the subduction and exhumation history within submillimetre-size grains. Advanced in situ techniques allow us to date zircon domains and to determine their trace element composition. We can thus acquire a detailed knowledge of the temperature-pressure-time paths that these extraordinary rocks have experienced. Zircon studies provide evidence that subduction and exhumation act at plate tectonic speeds of 1-3 cm/year.