This review of electron diffraction theory and technique emphasizes its scope and limitations in the study of lubrication. The first section describes important areas of wear and friction studies where the electron diffraction method may be used, with examples chosen from the literature. Included in the discussion are: (a) monolayers, liquid lubricants, and greases, (b) polished and amorphous films, (c) the relation to wear of the crystallite size of surface phases, (d) possible relations between wear and the orientation of surface films, (e) solid lubricants, (f) mechanism of extreme pressure lubrication, and (g) the identification of wear debris.