For many years controlled shot peening (CSP) was considered as a surface treatment of questionable benefits. This impression was f~~el led by contradictory results from fatigue experiments [1,2]. It is now clear that the performance of CSP in terms of fatigue depends on the balance between its beneficial (compressive residual stress and work hardening) and detrimental effects (surface roughening) [3,4]. Hence, in order to achieve a favourable fatigue performance, the role of those effects has to be analysed and understood. To achieve such undertaking it is essential to consider their interaction with other parameters such as the nature of the target material and the loading conditions. This work brings together two micromechanical models, (i) for notch sensitivity [5] and (ii) for fatigue life [6]. The former assesses the effect of surfxe roughening, whilst the latter incorporates the residual stress distribution ancl work hardening on fatigue life calculations. Combination of the two models allows the determination of the residiral stress distribution to meet specific improvements in fatigue life (improvement life factor, ILF). Using the ILF methodology, the effects of CSP can be scrutinised against stress level, surface roughness and ILF value.