  The FCC-ee aims to improve on electroweak precision measurements, with goals
of 100 keV on the Z mass and width, and a fraction of MeV on the W mass.
Compared to LEP, this implies a much improved knowledge of the centre-of-mass
energy when operating at the Z peak and WW threshold. This can be achieved by
making systematic use of resonant depolarization. A number of issues have been
identified, due in particular to the long polarization times. However the
smaller emittance and energy spread of FCC-ee with respect to LEP should help
achieve a much improved performance.
