SUMMARYThe principal advantages of the high voltage electron microscope are first described. Examples are then quoted of various topics in the study of materials in which these advantages are being applied at present. The most obvious improvement, that of greater penetration is utilized in the study of thicker specimens, useful particularly for observation of multiphase solids, experiments in the microscope and environmental cells. The reduction in ionization damage with increasing energy allows observation of many susceptible materials, while the onset of displacement damage interferes with certain other observations. However, the study of displacement damage in nuclear materials using the very fine flux conditions in the HVEM has proved to be of great technological importance. Finally it is shown how a particular high voltage effect, the critical voltage, may be utilized.