This paper describes the use of the back-scattered electron (BSE) mode of scanning electron microscope (SEM) operation for mineralogical and earth sciences applications. Such operation requires an efficient detector system, and of the types presently available, the one described here (an annular silicon diode) gives comparable if not superior results to the commercially available scintillator-photomultiplier systems. Other BSE detectors constructed from photodiodes are also found to give good results and are inexpensive and simple to manufacture. Several aspects of BSE image quantification are considered. It is shown how atomic number contrast (which arises from the dependence of the BSE coefficient on composition) may be calculated, and the BSE coefficients for a wide range of common minerals are presented. Atomic number contrast gives rise to only a small proportion of the total image signal, and in order to remove other sources of contrast, special attention should be paid to specimen preparation. Carbon coating is found to be the best method of preventing the specimen from charging because unlike other methods (e.g., gold coating, environmental cell) it does not interfere with image detail or X-ray analysis. The paper also shows how compositional information and mineral proportions may be obtained from atomic number contrast images using a multichannel analyzer.