The basic components of an analytical electron microscope are described, and the types of information obtainable from a modem instrument are listed. These include; an image of the sample at high magnification, diffraction patterns from crystalline material in the sample, analysis of the surface layer, and images of the sample topography. Earlier instruments were designed to produce images, rather than to analyze the specimen, and design improvements leading to both functions being available in the same instrument are described. Very good vacuum systems are required to prevent the specimen being contaminated by carbon, and improvements in design are still to be expected. Some current figures for resolution, detectable mass and the optimal specimen thickness are quoted. -- AATA