AbstractTheoretical and experimental work on the atomic structure of grain boundaries in metals and ionic crystals is reviewed critically. The dislocation and O-lattice models are described and their usefulness is assessed with reference to a worked example. The models derived from computer simulations of the atomic structures of grain boundaries are described and critically assessed, particularly with regard to their predictive capacities. Key transmission electron microscope observations of lowenergy boundary planes and grain-boundary dislocations are discussed. The difficultles in interpreting high-resolution electron-microscope images of grain -boundary atomIC structures are described and some recent results are discussed in the light of these remarks and computer calculations of grainboundary atomic structures. Finally, an important result from X-ray diffraction studies of grain boundaries is analysed.