X-ray stress analysis is a nondestructive method enabling one to measure residual and loading stresses in polycrystalline materials. Known as a difficult method for several decades, the introduction of microprocessors allowed the automation of the measuring procedure and the subsequent storage and computation of diffraction data. Also the availability of position sensitive counters increased the efficiency of the method. Nowadays a residual stress or loading stress determination can be a problem of a few minutes if the time required for installation and alignment of the equipment is neglected. As the penetration of the X-ray beam into the investigated surface of polycrystalline materials is low and only a few micrometers the obtained information is that of a surface stress state. A further specialty of the method is the selective nature of X-ray stress analysis: lattice strains are measured in certain crystallographic directions. Therefore the elastic anisotropy of the single crystal has to be taken into account. If second phases are present in the investigated sample, also the effect of heterogeneity contributes to the stress analysis. Therefore, the “X-ray elastic constants” had to be introduced.