A review covering the last ten years of research on the structure of metallic crystal grain boundaries, including epoch-making observations, presently under way, of lattice images using electron microscopy, is prsented. While representative studies carried out in the 1970's on ordered grain structures were limited by the analytical techniques available, the role in alloy design in future investigations is emphasized. Visual models and computer simulations, as methods for analysis of grain boundary atomic configurations, and also information gained by lattice imaging microscopy are explained, and the importance of future studies on grain segregation structures is shown.