This is a tutorial review covering the crystallography of bicrystals, coincidence concepts, relaxation mechanisms, principles and main results of computer modelling, and the description of high angle grain boundaries in terms of periodic arrays of either dislocations or ordered sequences of coordination polyhedra in the boundaries. Structure - property relations in grain boundaries are reviewed in the light of existing models, with special emphasis on the segregation of impurities at grain boundaries and its effects on boundary structure as indicated by computer modelling