The technical concept of the scanning ion microscope (SIM) at GSI, which has now reached a resolution of 0. 5 mu m, are presented together with some details of general interest like diagnostics of very weak microbeams, beam scanning and switching. Results of past experiments concerning secondary electron imaging, energy loss imaging and the use of the microbeam for microlithography are reported as well as the special problems of operating a SIM at a pulsed accelerator. Advantages of a SIM compared to other microscopes are discussed.