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The three Fine Guidance Sensors (FGSs) on-board the Hubble Space Telescope (HST) are the first shear interferometers to be operated from a space-based orbiting platform. In normal HST astrometric observations, two FGSs are used for guiding and maintaining the pointing of the HST while the third performs astrometry. HST/FGS astrometric performance yields positional measurements accurate to better than 3 milliseconds of arc and can resolve point sources with separations of as little as 10 milliseconds of arc. This paper discusses the astrometric performance of FGS3 in its two operating modes, POSITION and TRANSFER SCAN. For POSITION mode this includes its ability to acquire and track stars, the precison of this "fine-lock" data, the limiting magnitudes of measurable stars, and the post-observation data processing techniques used to remove both short and long term HST pointing instabilities and characteristics. We also explore temperature effects. For TRANSFER SCAN, the "S-curve" is described and a discussion of its measurement, post-observation data processing, and limiting magnitudes of science objects is included.
