Shot peening is a post-manufacturing surface treatment process that is regularly used to introduce advantageous compressive residual stresses just below the surface of a component to improve its fatigue life. ESDU 19003 provides an introduction to the design and use of shot peening to improve the fatigue strength of helical tension and compression steel springs. A guide to the selection of suitable spring materials and an overview of shot peening, as well as a brief overview of different surface treatments such as honing or presetting, is given. Also discussed is the effect of applying a pre-stress during shot peening, the influence of shot peening intensity and the percentage coverage, and the effect of relaxation applied after shot peening on the compressive residual stresses. During heat treatment it is possible that some spring steels can experience decarburisation just below the surface of the spring which may reduce the amount of compressive residual stress. Data are presented, in graphical form, to show the effect of applied pre-stress, decarburistion, fatigue stress ratio and a corrosive environment on the fatigue strength of some spring steels.