The purpose of this experiment was to determine the role of residual stresses in .fatigue strength independent of other factors usually involved when residual stresses are introduced. It consisted of an investigation of the influence of residual stresses introduced by shot peening on the fatigue strength of steel (Rockwell C hardness 48) in unidirectional bending. Residual stresses were varied by peening under various conditions of applied strain. This process introduced substantially the same amount and kind of surface cold working with residual stresses varying over a wide range of values. Good correlation was found between residual stresses and fatigue strength'tending to' substantiate ,the concept of superpositioning of residual stresses and load stresses. It was also found that shot peening of steel of this hardness is beneficial primarily because of the nature of the macro residual stresses introduced by the process. There is no gain attributable to "strain hard ening" for this material. An effort was made to explain the results on the basis of three failure criteria: distortion energy, maximum shear stress, and maximum stress.