ESDU 87016 presents experimental data drawn from the literature that show the effect of relative scale and the influence of thickness on fatigue strength. It is found that in the absence of a stress gradient there is no discernible trend with size, and the data show that other factors such as surface finish are of greater significance. However, where there is a gradient, either due to asymmetric loading (for example, reversed bending) or the presence of a stress raiser, the data show the larger specimens to have significantly lower fatigue strengths. A similar trend is seen in geometrically similar welded joints, and this is explained from fracture mechanics considerations although calculations on that basis are found to predict a greater strength than is usually found experimentally. For welded joints, worked examples illustrate how the data for the effect of thickness may be used to estimate the fatigue strength using other ESDU correlations of data drawn from laboratory tests on small specimens. For all the data presented, details of the specimens are tabulated.