The fine guidance sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGS's photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of its field of view, (2) determine the true relative orbits of close faint binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mas M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.