Strain-controlled tension-compression fatigue tests were performed on the ASTM pressure vessel steels, A302 and A225, to study the damage processes that lead to failure in low cycle fatigue. The measurements of static property changes in partially cycled specimens, and hysteresis loop effects obtained during cycling, were utilized to reflect the pattern of damage accumulation. In addition, strain cycling tests were conducted on these materials to assess the applicability of the relationship Nf=εF−ε0εTR2 as proposed by Manson [4] and Coffin [5] and modified by Sachs, et al. [6]. The experimental data obtained were in good agreement with the failure life and the effect of mean strain as predicted by the foregoing equation. Accordingly, a positive mean strain (prestrain in tension) reduces fatigue life, since the fracture strain available for cycling is reduced by the amount of the prestrain. The damage studies indicated, however, that this equation cannot be used to describe the progress of damage during strain cycling. Rather, it appears that damage is governed by at least two, possibly interdependent, processes; namely, the loss of available ductility due to strain hardening and the formation and growth of cracks which finally determine failure. Both processes are reflected in the remaining ductility after partial cycling. At present, it is not clear how the two processes combine to yield the experimentally confirmed relationship, Nf=εF−ε0εTR2.