The magnitudes am directions of the frictional forces must be considered in any fracture mechanics analysis of crack growth under fretting fatigue conditions. A simple nonlinear friction model has been developed which relates the waveforms of the frictional forces to those of the applied forces when fretting occurs. It is shown that the waveforms are different in shape and that periods of dwell and phase shifts may occur in the frictional forces. These differences can significantly affect the rate of growth of cracks and hence life prediction.