  Knowledge of the beam energy at LEP2 is of primary importance to set the
absolute energy scale for the measurement of the W-boson mass. The beam energy
above 80 GeV is derived from continuous measurements of the magnetic bending
field by 16 NMR probes situated in a number of the LEP dipoles. The
relationship between the fields measured by the probes and the beam energy is
calibrated against precise measurements of the average beam energy between 41
and 55 GeV made using the resonant depolarisation technique. The linearity of
the relationship is tested by comparing the fields measured by the probes with
the total bending field measured by a flux loop. Several further corrections
are applied to derive the the centre-of-mass energies at each interaction
point. The beam energy has been determined with a precision of 25 MeV for the
data taken in 1997, corresponding to a relative precision of 2.7x 10^{-4}.
Prospects for improvements are outlined.
