The fatigue crack initiation life (cycles to obtain a 0.25-mm fatigue crack) was estimated for butt welds using strain-controlled fatigue concepts. Key developments which facilitated these estimates were the assumption of(K f ) m a x conditions (the largest value of Kf possible for a given weld shape), the use of computer simulation methods which modeled cyclic hardening and softening as well as mean stress relaxation effects, and the use of the fatigue properties of the actual weld zone in which the initial notch was located. The initiation life was found to be very sensitive to changes in Kf, but rather insensitive to strength level. The importance of residual stresses and mean stress varied with material as did the fraction of total life devoted to crack initiation. Mild steel (ASTM Specification for Structural Steel (A 36)) high strength, low alloy (ASTM Specification for High-Yield Strength, Quenched and Tempered Alloy Steel Plate, Suitable for Welding (A 514)) steel and aluminum alloy welds were considered.