Atomic Clock Ensemble in Space (ACES) is an ESA mission designed to test the Einstein's Equivalence Principle with high-performance atomic clocks in space and on the ground. Installed onboard the International Space Station, the ACES payload will generate a clock signal with fractional frequency instability and inaccuracy of 1 − 3 · 10 −16. Two links operating on microwave (MWL) and optical (ELT) frequencies will allow comparing the ACES clocks with the best atomic clocks on the ground in a global network. Space-to-ground and ground-to-ground comparisons will provide tests of Einstein's theory of general relativity, at the same time developing applications in geodesy and time & frequency metrology. The ACES flight model is close to completion. The cold atom clock PHARAO has been tested and delivered for integration in the ACES payload. The ELT link has been completed. MWL and the active hydrogen maser SHM are presently under test. System level tests have already started and will continue all along 2017. This paper will present the recent progress of the ACES mission and discuss future perspectives for testing fundamental physics with clocks in space. 1 ACES Mission Elements Proposed to the European Space Agency in 1997, the Atomic Clock Ensemble in Space (ACES) mission relies on PHARAO, a clock based on laser-cooled caesium atoms, to generate a high stability and accuracy time reference in space 1,2,3. The free fall conditions are crucial for PHARAO to reach or even surpass the performance of the best atomic fountain clocks on ground, while keeping a very compact volume, small mass and power consumption. Installed onboard the International Space Station (ISS), at the external payload facility of the Columbus module, the ACES payload distributes its time scale to ground clocks by using two independent time & frequency transfer links, a link operating in the microwave domain (MWL) and the ELT (European Laser Timing) optical link. On the ground, a network of MWL ground terminal and satellite laser ranging stations provides the physical interface between the ACES clock ensemble and atomic clocks on ground. The ACES payload is shown in Fig. 1. It has a volume of about 1 m 3 , for a mass of 230 kg and a power consumption of 450 W. The main onboard instruments are the cesium clock PHARAO and the active hydrogen maser SHM. The PHARAO clock reaches a fractional frequency stability of 1.1 · 10 13 / √ τ , where τ is the integration time expressed in seconds, and an accuracy of a few parts in 10 16. SHM is the ACES flywheel oscillator, also providing the frequency reference needed for the onboard characterization of the PHARAO clock stability and accuracy. PHARAO and SHM 100 MHz