The importance of the mechanical stability of oxide scales in the corrosion resistance of metallic components operating at high temperature is discussed. The mechanisms of scale fracture due to stresses generated during growth, applied directly or by thermal cycling, are described. Methods of measuring these mechanical propertiesin situhave been described and measurements of their fracture toughness produced. The influence of defects in the scale on fracture behaviour is discussed, together with methods whereby the effect of small irregular-shaped defects can be quantitatively assessed. Finally, the influence of strain rate on scale fracture is summarized, and by comparing the yield and fracture behaviour of surface scales it is suggested that the mechanical behaviour of these scales over a wide range of operating conditions can be predicted.