The role of grain surfaces and interfaces in nanocrystalline materials is investigated by Xray diffraction (XRD). Two apparently distinct aspects show the importance of the interaction of a single grain with the environment in which it is placed: the role of grain-grain interaction in determining the residual stress in a polycrystalline thin film and the surface effects in single powder grains on the macroscopic average cell parameter determined by powder diffraction. A brief theoretical introduction of the phenomena and their possible modelling is presented together with a discussion of practical examples.