X-ray diffraction residual stress measurements are one of industryâ€™s most versatile materials characterization tools. The procedure involves placing a known strain on a test sample and calculating the residual stress that would be required produce it, assuming a linear elastic distortion of the crystal lattice. This article describes the two most common x-ray diffraction measurement techniques based on the plane stress elastic model. It explains the difference between single and two-angle diffraction methods, and identifies potential sources of measurement error for each. The article also presents several application examples.