Many experiments indicate that mechanical properties of polycrystal-line materials are strongly affected by the grain boundary (g.b.). In this paper, the theoretical and experimental work on g.b. structures and their effect on mechanical properties of polycrystalline materials is briefly reviewed. In the first part of this paper, the different models proposed for describing the g.b. structures are introduced, including the dislocation model for describing small angle g.b., the coincidence site lattice (CSL)...