  High-energy muons can travel large thicknesses of matter. For underground
neutrino and cosmic ray detectors the energy loss of muons has to be known
accurately for simulations. In this article the next-to-leading order
correction to the average energy loss of muons through bremsstrahlung is
calculated using a modified Weizs\"acker-Williams method. An analytical
parametrisation of the numerical results is given.
