It is proposed that thermal effects govern friction in high speed unlubricated contact. The general principle can be found from a manipulation of Blok’s flash temperature theory to give the coefficient of friction as a dependent parameter. The principal factors controlling friction, provided the conditions are sufficiently severe, appear to be the decomposition temperature, hardness and thermal properties of the materials. The same mechanism appears from a numerical model of thermal effects in sliding. The model is compared with several sets of experimental results in different configurations.