Summary Fretting is examined in its engineering aspects and then considered on a more theoretical basis. The mechanism of fatigue initiation by fretting is discussed, and also some of the methods proposed for the prevention of fretting. Some attention is also given to the fatigue life of joints when fretting does not occur. The difficulties that have been encountered in understanding fretting and assessing its consequences are thought to have a rather special significance. It is considered that there is now sufficient knowledge to put the whole subject tentatively on a rational basis, but more research is required to remove some of the uncertainties that still remain.