We have developed a scanning tunneling microscope (STM) capable of operating at temperatures as low as 0.4 K and fields as high as 8 T. We have used this STM to study the energy gap of the high- Tc superconductors La-Sr-Cu-O and Y-Ba-Cu-O. We find that the reduced gap for these oxide superconductors falls in the range 3 < 2A/kB Tc < 7, for polycrystalline, single-crystal, and thin-film samples. We have also simultaneously imaged the surface topography and superconducting energy gap for thin films of the granular superconductor NbN. We occasionally see regions with smaller best-fit gaps that correlate with surface topographical features, but have been unable so far to image flux vortices. © 1988, American Vacuum Society. All rights reserved.