The low-pressure synthesis of rather pure nanocrystalline diamond films from fullerene precursors suggests that for a small enough grain size the diamond structure may be energetically preferred over graphite. Because of the small grain size of typically about 15 nm in these films, a significant fraction of the carbon atoms is situated in the grain boundaries (GBs). The surprisingly high wear resistance of these films even after the substrate is removed and their high corrosion resistance suggest that the grains are strongly bonded. Grain-boundary carbon is also believed to be responsible for the absorption and scattering of light in these films, for their electrical conductivity, and for their electron-emission properties. In spite of all these indications of a critical role played by GB carbon in achieving the remarkable properties of nanocrystalline diamond films, to date the atomic structures of the GBs are essentially not known.It is well-known that the electronic and optical properties of polycrystalline silicon films are significantly affected by the presence of GBs. For example GBs can provide active sites for the recombination of electron-hole pairs in photovoltaic applications. Also, in electronic devices such as thin-film transistors, GBs are known to play an important role. Because of silicon's strong energetic preference for sp3 hybridization over other electronic configurations, the structural disorder in silicon GBs is accommodated by a distortion of the tetrahedral nearestneighbor bonds and in the extreme by the creation of dangling bonds—that is, of three-coordinated Si atoms each having one unsaturated, bound electron in an otherwise more or less tetrahedrally coordinated environment.