The existing fatigue strain versus life (S-N) data for materials used in nuclear power plant components have been compiled and categorized according to material, loading and environmental conditions. Statistical models have been developed for estimating the effects of the various service conditions on the fatigue life of these materials. The results have been used to estimate the probability of initiating a fatigue crack. Data in the literature were reviewed to evaluate the effects of the size, geometry and surface finish of a component on its fatigue life. Fatigue S-N curves for components have been determined by adjusting the probability distribution curves of smooth test specimens for the effect of mean stress and then applying design margins to account for the uncertainties that arise because of component size, geometry and surface finish. The significance of the effect of the environment on the current code design curve and on the proposed interim design curves published in NUREG/CR-5999 is discussed. Estimations of the probability of fatigue cracking in sample components from boiling water reactors and pressurized water reactors are presented.