[en] The possibility of obtaining electron diffraction patterns from very small specimen regions combined with high resolution imaging by use of scanning transmission electron microscopy (STEM) allows the detailed study of small nuclei of reaction products or of crystal defects. The capabilities of this method have been extended by the design and construction of a TV system for the viewing and recording of microdiffraction patterns from our STEM instrument so that clear patterns can be obtained from regions as small as 10A in diameter. This system has been applied to the study of initial stages of oxidation of chromium films, revealing the presence of very small oxide nuclei and identifying these crystals as having a previously unsuspected spinel structure. The further stages of growth of oxides on chromium are being investigated. Initial results have also been obtained on the surface structure of oxides such as MgO. The extension of previous work on the diffraction from, and imaging of crystal surfaces by the use of medium-to-low energy electrons (15 to 1 keV) has allowed a much more complete understanding of the contrast-producing mechanisms. Application to the study of pyrolytic graphite surfaces has given a clear picture of the mosaic structure and defect distribution and provided a basis for the more reliable and quantitative general use of these techniques in surface structure analysis