Electron diffraction has become established as a research tool for studying the structure of surface layers and thin films, and it has now been used moderately extensively for this purpose for more than a third of a century. Yet its field of usefulness is not sharply separated from that of x rays. Both require material of considerable thickness. The ordinary electron-diffraction techniques are not sensitive to a single layer of atoms on a crystal surface. This is precisely the sensitivity required to make electron diffraction an indispensable research tool. It is needed in surface physics and surface chemistry; in the most important application, it is needed to understand catalysis. Lacking this sensitivity, as it does now, electron diffraction is not a serious competitor of x-ray diffraction in general usefulness, and it has no unique field as does neutron diffraction. It is often available as an occasionally used attachment of an electron microscope.