A brief survey is given of the methods of examining opaque specimens in the electron microscope. The principle of the scanning electron microscope is described and mention is made of previous instruments. The design of such microscopes is discussed and the theoretical limit on performance is determined. The optimum scanning-beam voltage, beam current and scanning speeds are given.A recently built electrostatically focused scanning electron microscope for opaque specimens is described. This microscope has several new features, including:(a) The use of high-energy electrons to reduce the effects of surface contamination of the specimen.(b) Oblique scanning of the specimen, so that the image contrasts are formed by the topography of the etched surface of the specimen.(c) Direct amplification of the electron beam with an electron multiplier having beryllium-oxide-coated dynodes.(d) Direct viewing at low magnifications on a cathode-ray tube with a long-persistence screen, and photographic; ecording from the screen at high magnifications.Examples of micrographs of aluminium taken with the instrument are included and these show a resolution of about 500 Å. It is concluded that higher resolutions could be obtained with this instrument if improved magnetic shielding and astigmatism correction were to be incorporated.Some measurements on the reflection of high-energy electrons are described in an Appendix.