It is the purpose of this paper to discuss the influence of primary recoil spectra on microstructural evolution in irradiated metals. In particular the consequences are explored of the recoil sensitivity of a transient population of vacancy loops formed directly in the vacancy rich cores of cascades on the evolution of the damage at low and high doses. The importance of the defect motion in the thermal spoke created by the displacement cascade is emphasised, and the dependence of vacancy loop production on material and irradiation parameters is described. The influence of vacancy loops on void swelling and irradiation hardening is discussed by reference to the predictions of the rate theory approach to physical modelling of the microstructural evolution. Finally, the consequences of high energy recoils on preexisting features of the microstructure is reviewed.