X-ray diffraction is a very powerful technique, since it allows acquiring different kinds of complementary information. First, the stresses of the major phases of the material can be derived from the peak positions measured on each constituent. This permits also evaluation of the mean macro-stresses. Second, the plastic deformation of the analysed phases can be characterised by the shape of the diffraction profiles. Third, the areas of the diffraction peaks, which represent the diffracted intensity, permit determination of the volume fraction of the different phases. Fourth, a background intensity increase is often observed during service, which is due to diffuse scattering of X-rays. This increase in background can define some damage induced by fatigue or wear. X-ray diffraction can thus be used to analyze and improve the mechanical behaviour of materials. In this paper, three experiments are presented which allowed developing new enhanced materials. The first example is about the improvement of the pitting behaviour of carbo-nitrided steel layers. In the second experiment, a global X-ray method will be presented which allowed a better characterisation of the material and manufacturing process parameters of steel springs. The third example is about ceramic coatings used in dental applications. In that last case, neutron and high energy synchrotron measurements have been carried out to evaluate the stress existing at the metal/ceramic interface.