An attempt is made to show that the highly localized temperature and associated thermal stress gradients in front of active slip planes, resulting from the conversion into heat of the work in slip of the resolved shear stress, are of sufficient severity to account for the initiation of micro‐cracks parallel to the slip planes if, as in fatigue, slip under repeated stress cycles is concentrated into ``striations.'' The effect of the thermal characteristics of the metal on its fatigue performance is discussed.