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.

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