A broad investigation of the structure and properties of grain boundaries has been conducted. The approach was mainly experimental, and extensive use was made of high resolution methods of investigating grain boundaries in specimens containing boundaries of controlled geometry. Progress was made in the following areas: (A) methods for preparing thin-film specimens containing boundaries of controlled geometry; (B) study of the fine structure of high angle boundaries with emphasis on the detection of relaxations into various fit-misfit structures; (C) structure of low angle grain boundaries; (D) faceting and the preferred crystal orientation of grain boundaries; (E) grain boundaries as sinks for point defects; and (F) diffusion along grain boundaries.