RESULTS of an investigation into the effect of shot-peening variables and the resulting residual stresses on fatigue life are reported in this paper. Leaf springs were the simple specimens heat-treated, cold worked, and tested in this study.Some of the conclusions reached are: 1. There is a minimum shot velocity for each shot size to obtain best fatigue life, and this value is much lower than that normally used. 2. Exposure time for this type of shot-peened specimen beyond some minimum value is wasteful and costly. 3. Shot size has little influence on fatigue life for these specimens. 4. Shot peening specimens while under tensile strain greatly increases fatigue life at 200,000 psi nominal stress over that of nonpeened or strain-free-peened specimens. 5. Shot peening these specimens gave residual compressive stresses 50% of yield strength, and these stresses can be increased to more than 50% by strain peening. 6. There is a direct correlation between surface residual compressive stress and fatigue life of these specimens.