This talk  will introduce the Future Circular Colliders project and its Feasibility Study. 
The FCC project features a 90 km long tunnel in the Geneva basin where an electron-positron collider 
crossing four relevant electroweak thresholds (Z, WW, HZ and tt) shall be installed first. A longer term 
energy frontier proton collider, defining the infrastructures, is envisaged to be hosted in the very same 
tunnel. The exquisite luminosity of the electron circular machine at each energy can challenge the 
electroweak observables precision consistency test and offer, in particular at the Z pole but not only 
there, a continuation of the exploration of the Flavour Physics case beyond the vibrant LHCb and 
Belle II programmes. We’ll review these Physics opportunities and discuss some of the related 
detector requirements and detector R&D.