Atomic clocks are an outstanding tool for the experimental verification of general relativity and more generally for fundamental astronomy (VLBI, pulsar timing, navigation, etc). Recent years have seen a rapid improvement in the performance of such clocks, promising new improved tests of relativity, in particular onboard terrestrial and interplanetary space missions. We present the scientific motivations of such tests taking the ACES Salomon et al. and SAGAS Wolf et al. (2009) projects as particular examples.
