Transmission electron microscopy is used to study the structure of grain boundaries (GBs) in NiO. tilt GBs in NiO are generally found to be faceted. Structural periodicities within facets have been observed at low and high misorientation. An effective reduced atomic density at the GB within a region of 0.5-0.8 nm in width is suggested by the defocus contrast behavior of high-angle tilt and twist GBs