The quality of diamond films deposited from the vapor phase was analyzed via transmission electron microscopy. Diamond films grown by different deposition processes and under various conditions were examined and the processing microstructure relationships were established. Defect structures are first reviewed here. Twinning was the predominant defect observed in all of the diamond samples, but stacking faults and dislocations were also found. It was found that a lower methane concentration resulted in a lower defect density (higher quality) in diamond films grown by microwave-plasma-enhanced chemical vapor deposition. The defect density in diamond films was also reduced if reverse bias was applied in a bias-controlled hot-filament chemical vapor deposition system, in contrast with the high defect density which occurred under the forward bias condition. Finally, the imperfection density was substantially reduced if diamond films were grown at higher substrate temperatures in an oxyacetylene torch. © 1991.