Nanometre-sized or nanocrystalline materials are solids with very small grain size (5–10 nm) and, therefore, a considerable fraction of atoms located in the disordered interfacial structure, giving rise to novel physical properties and potential technological applications. X-ray diffraction, Mössbauer spectroscopy and positron lifetime measurements indicate a wide interatomic distance distribution in the interfacial structure. The thermal, elastic and magnetic properties, the internal friction behaviour, as well as the high diffusivities and reactivities are discussed in terms of the present results on nanocrystalline metals, semiconductors and metal oxides. Some future prospects are outlined.