The mixed fretting regime (MFR) has been proven to be the most dangerous regime for crack nucleation and service failure. It can be identified from the evolution of the "tangential toad-displacement" loops, i.e. when there is a variation in the shape of the loops. Usually elliptical (partial slip) and quasi-rectangular (gross slip) loops are encountered in MFR. This regime has been obtained in fretting wear and fretting fatigue tests on numerous metal-base alloys. In contrast with the classic stick-slip condition in Mindlin's elastic theory, MFR depends strongly on the normal load, the imposed amplitude and material properties such as elongation. During fretting testing under MFR, the material can fail due to overstraining or overstressing and the location of the maximum varies. For ductile materials, a competition exists between the formation of the tribologically transformed structure, particle detachment and the nucleation of fatigue cracks. In the case of brittle materials, MFR is generally avoided due to the rapid formation of debris and the establishment of a third body layer in which easier velocity accommodation favours the gross slip condition. A mechanical and material approach is used to analyse the specific behaviour of materials submitted to MFR. The study of fretting crack nucleation and propagation is emphasized.