Residual stresses are those that remain in the material after external loads have been removed. To satisfy static equilibrium, they are self-equilibrating. Residual stresses are generated as a response of the material to inhomogeneous permanent inelastic strains. The latter will induce elastic strains –residual strains– to fulfil strain compatibility, which in turn will produce residual stresses. Therefore, residual stresses are associated to elastic residual strains. These stresses may cause unexpected failures if not properly considered, because they add up to applied stresses. This is particularly important in the case of high-risk components subjected to subcritical crack growth processes –fatigue or environmentally assisted cracking–; for example, in the transportation industry (automotive, aeronautics and railways), where residual stresses are essential to estimate lifetimes in a reliable way. There are several methods to measure residual stresses, including destructive and non-destructive techniques. From the non-destructive ones, diffraction methods (X-rays and neutrons) are the most widely used for a quantitative assessment of residual stresses. X-rays are very useful for surface and subsurface measurements (with material removal). However, if a bulk measurement is needed, neutrons (along with high-energy synchrotron radiation) are usually the best choice. They penetrate easily into structural materials (i.e., steel and aluminium alloys) and can provide truly non-destructive residual stress measurements inside large components. As an example, the influence of residual stresses, due to cold drawing, on the hydrogen embrittlement susceptibility of pearlitic wires used for prestressing concrete was investigated. Residual stresses were obtained by diffraction using laboratory X-rays, neutrons and synchrotron radiation and hydrogen susceptibility was evaluated through rupture times in NH4SCN tests.