Residual stress measurements using x-ray diffraction is a well established methodused within the industrial and academic community to verify the performance of differentprocesses for metallic materials. The measurement gives an absolute value of the stress statewhich can be used to design and optimize the process route to induce beneficial compressiveresidual stresses and avoid detrimental tensile stresses. Investigating the uncertainty andaccuracy of the measurement system, operator and the material is therefore of high relevanceboth from an industrial and scientific point of view. Round robin testing is an important way toquantify the uncertainties that could affect the quality of the measured results and hence how aprocess is optimized and tuned. Such an investigation allows the operator to understand andreduce variations. Current round robin test includes results from five different laboratories usingcomparable equipments located in Sweden, Finland, Germany and United States. This workfocuses on five shot-peened tool steel specimens produced with identical process settings.Additionally, an investigation of the repeatability of the system, influence of the operator,variations within the specimen, and the long time stability of the specimens has been measured.