An energy analyzer for electron beams, based on a retarding-potential method, has been studied both experimentally and theoretically. In this method a potential energy barrier at right-angles to the beam acts as a high-pass energy filter allowing electrons having energies greater than the barrier height to pass through while turning back electrons of lower energy. The potential barrier in the present study was created by applying a negative potential to an electrode consisting of a 600/inch hexagonal copper mesh or an electrode having a pair of single apertures in series (compound aperture). Two different compound apertures, of diameters 0.4 mm and 1.0 mm, were used. The retarding electrode was mounted between two grounded outer electrodes having openings centered on the axis for entrance and exit of electrons. The barrier height was adjusted by means of a small variable bias voltage between the electron gun cathode and the retarding electrode. Auxiliary electron lenses external to the retarding module were used to satisfy the conditions that the beam be normal to the retarding electrode and, for an imaging filter, that an image plane be focused at the retarding electrode. A beam having a narrow distribution of energies was used to calibrate the energy analyzer as a function of bias voltage for the three different configurations of retarding electrode. The calibration curves were then compared with the transmission curve for a beam having a broadened energy distribution. The feasibility of obtaining a filtered image was explored by observing the image of a fine mesh focused into an aperture of the retarding electrode. The experiments were carried out for a beam voltage of 15 kV. At this beam voltage the energy resolution is poor, being 3. 6 volts for the smaller compound aperture, 3. 7 volts for the mesh, and 5.2 volts for the larger compound aperture. Typically, electrostatic analyzers operate on a beam which has been decelerated to low energies before entering the analyzer, in which case the energy resolution is proportionately better. For example, at 150 V the above resolution figures would be 0.036, 0.037, and 0.052, respectively. In view of the potential for high energy-resolution and the attractive features of in-line, rotationally-symmetric systems, it is felt that the retarding-potential analyzer is a promising candidate for further development.